SSC JE CIVIL ENGINEERING
INTRODUCTION
Surveying is the art of determining the relative position of different objects on the surface of the earth by
measuring horizontal distance btw them, and preparing map to any suitable scale.
So, in this section we only measure distances only in horizontal plane.
OBJECT OF SURVEYING
THE aim of surveying is to prepare a map to show the relative position of objects on surface of the earth.
2. Geodetic Surveying
In Plane survey the curvature of earth is not taken in to consideration. The lines joining any two points is considered as to be straight, and the triangle formed by any three points is also assumed as plane triangle. The area considered below the 250 km2.
In geodetic survey the curvature of earth is taken into consideration in order to have high degree of precision. The lines joining any two points is considered as a curved line. The triangle formed by any three points is also assumed as spherical lines. The area considered above the 250 km2.
Topographical Surveys
Cadastral Surveys
City Surveys
Engineering Surveys
(1) Control survey
(2) Hand survey
(3) Topographic survey
(4) Engineering survey
(a) Reconnaissance survey
(b) Preliminary survey
(c) Location survey
(5) Route survey
(6) Construction survey
(7) Astronomic survey
(8) Mine survey
Information about both surface and sub surfaces acquired for assessing for different type of reserve like the minerals, rocks and also folds, faults and helps in determining the type of foundation required and which soil treatment is required.
This is done for depicting the land use efficiency, irrigation intensity, surface drainage, slope profiles, x contours.
It is done to prepare detailed drawing of projects involving roads, railways, dams, water supply design, reservoirs, bridges etc.
1) Reconnaissance survey
To explore site conditions and availability of infrastructures.
2) Preliminary survey
To collect adequate data to prepare plan / map of area to be used for planning and design.
3) Location survey
To set out work on the ground for actual construction / execution of the project.
To established boundaries of properties for legal purpose for fields, estates and houses, etc.
Surveys done for military purpose, provide strategic information for deciding the future course of action.
It is done to determine the natural features of a country.
It is done to gather the information about ancient monuments, towns, villages, kingdoms.
It is carried out to locate the premises, streets water supplies sanitary systems, etc.
To establish horizontal and vertical positions of control points.
To determine the latitude, longitude (of the observation station) and azimuth (of a line through observation station) from astronomical observation.
To plan, design, and laying out of route such as highways, railways, canals, pipelines, and other linear projects.
Surveys which are required for establishment of points, lines, grades, and for staking out engineering works (after the plans have been prepared and the structural design has been done).
Chain surveying is the simplest method of surveying in which the linear measurements are directly taken in the field and the angular measurements are not taken. This type of surveying is used over small and levelled area.
Here both linear and angular measurements are made, former with made with tape or chain later with compass. This type e of survey useful for large project like dam or reservoirs.
Here elevation of different points are determined. Graduated staff and level, dumpy or automatic level is used.
In this method of surveying in which the horizontal and vertical distances of relative points are determined with the graduated staff with a transit telescope fitted with anallatic lenses.
Here observation and plotting is done simultaneously in field it is mainly used for small and medium scale mapping where great accuracy is not required.
I this surveying whole area is divided into a network of triangles to determine distances and relative positions of points spread over an area, by measuring the length of one side of each triangle and deducing its angles and length of other two sides by observation from this baseline.
EMD refers Electronic Device Measurement and in this method, distance are measured electronically using wave propagation, reflection and subsequent reception of the reflected wave.
It the combination of transit theodolite with electronic distance meter (EDM).It is also integrated with microprocessor, electronic data collector and storage system. It is used to measure sloping distance of object to the instrument, horizontal angles and vertical angles.
Data collected from total station can be downloaded into computer/laptops for further processing of information.
Here information about the land is determined by using satellite-based navigation system and GPS.
According to the first principle the whole area is first enclosed by main station and survey line. The area is then divided into a number of parts by forming well-conditioned Triangle.The purpose of this process of working is to prevent accumulation of errors during this procedure if there is any error in the measurement of any side of a triangle then it will not affect the whole work. The error can always be detected and eliminated.
Passometer- It count the number of steps automatically.
Perambulator- it is a wheel fitted with fork and handle. Wheel is graduated and shows a distance per revolution.
Angular measurement refers to magnetic bearing or horizontal angle taken by a PRISMATIC COMPASS or THEODOLITE.
The principle of chain surveying is TRIANGULATION this means that the area to be surveyed is divided into a number of small triangles that should be well conditioned.
Preferably, all the sides of a triangle should be nearly equal having each angle nearly 60° to ensure minimum distortion due to errors in measurement of sides and plotting.
Generally, such an ideal condition is practically not possible always due to configuration of the terrain and, therefore, an attempt should be made to have well-conditioned triangles in which no angle is smaller than 30° and no angle is greater than 120°.
The arrangement of triangles to be adopted in the field depends on the shape, topography, and the natural or artificial obstacles met with.
Chain survey recommended when
The ground surface is more or less level.
The area is to be surveyed.
A small-scale map is to be prepared.
The formation of a well-conditioned triangle is easy.
Chain surveying is unsuitable when
First, the area is crowded with many details.
The area consists of two many undulations.
The area is very large.
The formation of the well-conditioned triangle becomes difficult due to obstacles.
SURVEY STATION
MAIN SURVEY STATION
Station taken along the boundary of an area as a controlling point is known as the main survey station. the line joining the main station is called the main survey line.
Main Survey Station- A, B, C, D
Tie Station- E, F
Main Survey Line- AB, BC, CD, DA
Tie Survey Line- DE, FB
Base Line- AC
Check Line- BH, BG
TIE STATION OR SUBSIDIARY STATIONS
The station which is on the main survey line or any other survey line is known as a subsidiary station.
The line on which the framework of a survey is built is known as a baseline the long survey line which is run through the middle of the area to be surveyed.
The line is run to check the accuracy of the traverse consisting of a framework of triangles. It joins the apex point of a triangle to some fixed point on its base is known as the check line. it is taken to check the accuracy of the triangle sometimes this line help to locate interior details.
The lateral measurement taken from an object to the chain line is called offset. Offsets are taken to locate the object with reference to the chain line.
Perpendicular offsets
When the lateral measurement is taken perpendicular to the chain line they are known as perpendicular offset.
oblique offsets
oblique you upset is taken when the object is at a long distance from the chain line it is not possible to set up a right angle due to some difficulties.
Chainage is the horizontal distance as measured along with a combination of curves and straight lines (curvilinear) between two points. Chainage It is the distance of a well-defined point from the starting point. In chain surveying, it is normally referred to as the distance of the foot of the offset from the starting point on the chain line.
The operation of measuring the distance is termed as chaining/taping. The Point of Beginning of the line is usually denoted as 00+00 (The Starting Point) with stationing at least every 100 feet along the line denoted as 1+00 (for 100 feet)…2+00 (for 200 feet)…3+00 (for 300 feet), etc. where the first number represents the number of 100-foot stations and the digits after the (+) sign represents any remaining portion less than 100 feet.
For example 31+57.95 would represent 3,157.95 feet.
Metric chains
Metric chains are the most commonly used chain in India. These types of chains come in many lengths such as 5, 10, 20 and 30 meters.
The most commonly used is 20m and 30m chain.
The brass ring is placed at every meter.
The total length of the chain is marked on the brass handle at the ends.
20 meter chain 100 link @ 0.2m, Tallies after 10 links(2m). [0.2*10=2m]
30 meter chain 150 link @ 0.2m, Tallies after 25 links(5m). [0.2*25=5m]
Revenue Chain
The standard size of this type of chain is 33ft. The number of links is 16, each link being 2(1/16) ft. This chain is commonly used in the cadastral survey.
33ft = 10.06 m link @ (33/16 =62.87).
Gunter’s chain or surveyor’s chain
Gunter chain comes in standard 66ft. This chain consists of 100links, each link being 0.66ft or 7.92inches.
The length 66ft is selected because it is convenient in land measurements.
1 mile = 8 Furlongs = 80 Gunter chains
1 Acre = 10 square Gunter’s chains
10 Gunter chains = 1 Furlong
Engineer’s chain
This chain comes in 100ft length. It consists of 100 links each link being 1ft long.
At every 10 links, a brass ring or tags are provided for an indication of 10 links.
Readings are taken in feet and decimals.
Testing and Adjustment of Chain
As the chain is a metal made, it may undergo many changes due to temperature effect or human error and etc. So, for all lengths of the chain, a tolerance is given,
10m chain = + or – 3mm
20m chain = + or – 5mm
30m chain = + or – 8mm
Personal Errors
Wrong reading, wrong recording, reading from the wrong end of the chain, etc., are personal errors. These errors are serious errors and cannot be detected easily. Care should be taken to avoid such errors. Compensating Errors
The compensating errors are those which are liable to occur in both direction i.e (positive and negative) and finally hence tend to compensate are known as compensating error.
Hence, they are likely to get compensated when a large number of readings are taken.
They are directly proportional to √L
In chaining, these may be caused by the following: -
The incorrect holding of the chain: - The follower may not bring his handle of the chain to the arrow, but may hold it to one or the other side of the arrow.
Fractional parts of the chain or tape may not be correct if the total length of the chain is adjusted by insertion or removal of a few connection rings from one portion of the chain, or tape is not calibrated uniformly throughout its length. Graduations in the tape may not be exactly the same throughout.
During stepping operation crude method of plumbing (such as dropping of stone from the end of the chain) is adopted.
When chain angles are set out with a chain that is not uniformly adjusted or with a combination of chain and tape.
Cumulative Errors
The cumulative errors are those which occur in the same direction and tend to add up or accumulate i.e. either to make the apparent measurement always too long or too short.
In each reading, the error may be small, but when a large number of measurements are made, they may be considerable, since the error is always on one side.
Examples of such errors are:
Bad ranging
Bad straightening
Temperature variation
Variation in applied pull
Non-horizontal it
They are directly proportional to L.
Positive errors
(making the measured lengths more than the actual) are caused by the following:-
The length of the chain or tape is shorter than the standard, because of bending of links, removal of too many links in adjusting the length, ‘knots’ in the connecting links, cloggings of rings with clay, temperature lower than that at which the tape was calibrated, shrinkage of tape when becoming wet. The slope correction is not applied to the length measured along the sloping ground.
The sag correction is not applied when the tape or the chain is suspended in the air.
Measurements are made along the faulty alignment.
The tape is in suspension or in high winds.
Negative errors
(making the measured lengths less than the actual) maybe caused because the length of the tape or chain may be greater than the standard because of the wear or flattening of the connecting rings,
opening of ring joints,
temperature higher than the one at which it was calibrated.
Applied more pull than the standard pull.
Tapes are used in surveying to take linear measurements. They are available in different lengths and can be made of different materials. There are 5 types of tapes available in surveying for linear measurements and they are as follows
Linen Tape.
Metallic Tape.
Steel Tape.
Synthetic Tape.
Invar Tape.
Linen tape, also known as cloth tape is a varnished strip made of closely woven linen and it is varnished to resist moisture.
15mm wide. Available in 10m,15m.
Used for taking offset for ordinary work.
Metallic Tape, when linen tape is reinforced with brass and copper wire to make it durable, then it is called metallic tape.
available in different lengths of 10m, 15m, 20m, 30m, and 50m.
These are used for survey works such as topographical survey works.
Steel tape, is made of steel or stainless steel.
It consists of a steel strip of 6mm to 16mm wide.
Synthetic tapes are made of glass fibers coated with PVC.
These are light in weight and flexible.
Synthetic tapes may stretch when subjected to tension. Hence, these are not suitable for accurate surveying works.
Invar tapes,
36% of nickel and 64% of steel.
6mm wide strip and is available in different lengths of 30m, 50m, 100m.
The coefficient of thermal expansion of the invar alloy is very low. It is not affected by changes in temperature. Hence, these tapes are used for high precision works
Accuracy Sequence of tapes
Invar Tape > Steel Tape > Metallic Tape > Linen Tape
Ranging rod is a surveying instrument used for making a LINE STRAIGHT. And also marking the position of stations, and for sightings of those stations. Made with the well-seasoned wood such as teak, pine or deodar, and GI pipes. Diameter- 25mm Length- 2 meter Alternate BLACK & WHITE Strip of length 0.2 meter. The lower end is pointed and provided with the iron shore.
Offset Rods
These are similar to ranging rods except at the top where a stout open ring recessed hook is provided, It is also provided with two short narrow vertical slots at right angles to each other, passing through the center of the section, at about eye level. It is mainly used to align the offset line and measuring the short offsets. With the help of the hook provided at the top of the rod, the chain can be pulled or pushed through the hedges or other obstructions, if required. Offsets may also be made in the field with the help of a cross-staff or optical square.
Arrows
When the length of the line to be measured is more than a chain length, there is a need to mark the end of the chain length. Arrows are used for this purpose. Arrows are made up of 4 mm diameter steel wire with one end sharpened and another end bent into a loop. The diameter of the loop is 50mm, Total length is 400mm.
Pegs
Wooden pegs are used in measuring the length of a line to mark the endpoints of the line. The pegs are made of hardwood of 25 mm × 25 mm section, 150 mm long with one end sharp. When driven in the ground to mark station points, the project about 40 mm. Clinometer is an instrument used for measuring angles of slope (or tilt).
Cross-Staff
It is essentially an instrument used for setting out right angles. In its simplest form, it is known as Open Cross-Staff (a). It consists of two pairs of vertical slits providing two lines of sight mutually at right angles. Another modified form of the cross-staff is known as French Cross-Staff (b). This consists of an octagonal brass tube with slits on all eight sides. This has a distinct advantage over the open cross-staff as with it even lines at 45° can be set out from the chain line. The latest modified cross-staff is the Adjustable Cross-Staff (c). It consists of two cylinders of equal diameter placed one above the other. The upper cylinder can be rotated over the lower one graduated in degrees and its subdivisions. The upper cylinder carries the Vernier and the slits to provide a line of sight. Thus, it may be used to take offsets and to set out any desired angle from the chain line.
Optical Square
Optical Square It is more accurate than the cross-staff.
It is a small and compact hand instrument and works on the principle of reflection. Generally, it is a 5 cm in diameter and 1.25 cm deep.
used for locating objects situated at larger distances. a metal cover to protect it from dust, moisture, etc.
It consists of a horizontal mirror (H) and an index mirror (l) placed at an angle of 45 degrees to each other.
The mirror H is half-silvered and the upper half is plain while the mirror I is fully silvered. There are three openings a, b and c on the sides.
Let AB is the chain line and it is required to locate an object O during the process of surveying. The optical square is held in such a manner that a ray of light from object O passes through slot c, strikes the mirror, gets reflected and strikes the silvered portion of the mirror H. After being reflected from H, the ray passes through the pinhole and becomes visible to the eye. The observer looking through the hole a can directly see the ranging rod at B through the un-silvered portion of the mirror H and he image of the ranging rod placed at O. Thus, when both the ranging rods coincide, the line OD becomes perpendicular to the chain line. If they do not coincide, the optical square has to move back and forth to get the correct position of D.
Ranging
The term ranging is used to establish a set of intermediate points on a straight line whose two ends have already been fixed on the ground, whereas Chaining means measuring the length of a straight line with chain or tape. Chaining a long line necessarily involves ranging. There are two methods of ranging-
When an intermediate ranging rod is fixed in a straight line by direct observation from an end station, the process is known as direct ranging.
When end station is not visible due to there being high ground between them intermediate ranging rod fixed on the line in an indirect way this method is known as indirect ranging or (reciprocal ranging)
Taping on a Sloping or Uneven Ground
If the slope of the ground is more than 3° or 1 in 20, there is a considerable difference between the slope distance and the horizontal distance. One of the following procedures may be adopted to determine the horizontal equivalent of the measured slope distance in such cases:
1. Direct method 2. Indirect method.
Direct method: In the direct method, also known as the stepping method, the horizontal equivalent of the slope distance is directly measured, as shown in Fig. It is more convenient to measure downhill than to measure uphill and, therefore, uphill measurement should be avoided as far as possible. In the case of downhill measurements, the horizontal distance is measured in steps as shown in Fig. 3.8a using the procedure explained for taping on the flat ground. For taping uphill as shown in Fig. 3.8b, the rear end of the tape is held at A' above A and the other end at B on the line AE, and the measurement proceeds upwards in a similar manner as above.
Indirect Method- When the slope is steeper than the steeping method is not suitable. In such a case, the horizontal distance is measured by the following method.
Measuring the slope with Clinometer.
By Applying for Hypotenuse allowance
By knowing the difference of level between two points.
1. In this method, the angle QPR can be measured by a clinometer or on the vertical circle of the transit.
Then. PQ= PQ cos Hypotenuse allowance is one of the indirect methods of ranging on uneven or sloping ground. In this method, a correction is applied at every chain length and at every point where the slope changes. This facilitates locating or surveying the intermediate points. Let α = the angle of slope of the ground.
AD = AB = 1 Chain = 100 links.
Then AC = (100 sec) α links and
BC = AC – AB
BC = (100 Sec α) – (100) links.
BC = 100 (Sec α- 1) links.
The amount 100 (sec α – 1) is known as a hypotenuse allowance.
3.Knowing the difference in the level
Suppose, A, B, C, and D are different points on sloping ground. The difference of level between these points is determined by a leveling instrument. Let the respective differences be h1, h2, and h3. Then the sloping distances AB, BC, and CD are measured. Let the distances L1, L2, and L3 respectively (Fig).
Obstacles in Chaining of a Line | Land Survey | Surveying
The three main obstacles in the chaining of a line are of the following types:
1. Chaining Free, Vision Obstructed
2. Chaining Obstructed, Vision Free
3. Chaining and Vision Both Obstructed.
1. Chaining Free, Vision Obstructed:
In this type of obstacle, the ends of the lines are not intervisible e.g. rising ground, hill or jungle intervening.
(i) Both ends may be visible from any intermediate point lying on the line such as in the case of a hill. The obstacle of this kind may easily be crossed over by reciprocal ranging and length measured by the stepping method of chaining. (ii) Both ends may not be visible from any intermediate point such as in the case of a jungle. The obstacle of this kind may be crossed over by the “Random line method”. 2. Chaining Obstructed, Vision Free
The typical obstacle of this type is a sheet of water, the width of which in the direction of measurement exceeds the length of the chain or tape. The problem consists in finding the distance between convenient points on the chain line on either side of the obstacle. (a) When the obstacle can be chained around, e.g. a pond, a thorny hedge, etc. (b) When the obstacle cannot be chained around e.g. a river. 3. Chaining and Vision Both Obstructed A building is a typical example of this class of obstacles. The problem, in this case, consists both in prolonging the line beyond the obstacle and finding the distance across it.
It is not always representing the actual distance of an object on the drawing.
The scale is the ratio of the distance of two-point on a sheet or map to the same distance of two-point in the ground.
Representative Fraction (RF)=(Distance beteen two point on sheet)/(Same distance on the ground)
Small Scale- 1 cm = 100 m
Medium Scale- 1 cm = 50 m
Large Scale- 1 cm = 10 m
Smallest scale is that whose denominator is larger value.
The scales are classified into four categories-
Plain Scale
Diagonal Scale
Scale of chords
Vernier Scale
Plain Scale
Plain Scale is one on which it is possible to measure two dimensions only. For example, measurements such as units and lengths, meters and decimeters, etc.
Diagonal Scale
On the diagonal scale, it is possible to measure three dimensions such as meters, decimeters and centimeters, units, tens and hundreds; yards, feet, and inches, etc. A short length is divided into a number of parts using the principle of the similar triangle in which sides are proportional. 1-1 represent 1/10 PQ
2-2 represent 2/10 PQ
9-9 represent 9/10 PQ
Scale of Chords
The scale of chords is used to measure an angle and is marked on either on a rectangular protractor or an ordinary boxwood scale.
Vernier Scale
A device used for measuring the fractional part of one of the smallest divisions of a graduated scale.
It usually consists of a small auxiliary scale that slides alongside the main scale.
Least count of the Vernier = the difference between smallest division on the main scale(S) and smallest division on the Vernier scale(v).
Least Count = S-v
Direct Vernier
It is directly calibrated in same direction of the main scale.
The Direct Vernier scale divisions are shorter than the Main Scale divisions. If it is required to read 1/nth part of the smallest division on the main scale, (n-1) main divisions are taken and divided into n equal divisions on the Vernier scale.
nv=(n-1) d
n = the number of divisions on the vernier.
v = the value of the smallest division on the Vernier scale.
d= the value of the smallest division on the Main scale.
Retrograde Vernier
It is directly calibrated in the opposite direction of the main scale.
In this type of the vernier, (n+1) divisions of the main scale are taken and divided into n divisions on the vernier scale.
nv=(n+1) d
(i) Vernier divisions are longer than the main scale divisions,
(ii) The graduations of the main scale are marked in the direction opposite to that of the vernier scale-one from right to left and the other from left to right.
The only advantage of a retrograde vernier is that the graduations are bigger than those of a direct vernier. But as it has to be read in the opposite direction, which is rather difficult, it is not commonly used.
Extended vernier nv=(2n-1) d
Double Vernier
With a simple vernier, readings can be taken in one direction only, but a double vernier is required when the graduations on the main scale are marked in both discussions from the common zero, such as at Abney’s level. In a double vernier, two simple Verniers are placed end to end forming one scale with the zero in the center. One is used for readings in the clockwise direction and the other for the readings in the anticlockwise direction. In the case of a vernier attached to the vertical circle of a transit theodolite which is divided into the quadrants, two sets of graduations are marked on a single vernier instead of providing a double vernier. In reading this vernier, only that set is used which increases in the same direction as the graduations on the quadrant which is being read. There are some other special forms of vernier such as an extended vernier used on the astronomical sextant, and the double-folded vernier used in compasses, etc.
Shrinkage Ratio=(Shrunk length)/(Actual length)= (Shrunk Scale)/(Origional Scale)
Shrinkage ratio / factor is < 1 or equal to 1.
Correction due to incorrect Tape Length
True length of chain=(True length±Errors)/(Standard length of chain)*Measured Length(ML)
Corrections in Chain Surveying
If the actual tape length is not equal to standard value then the correction should be applied.
The correction for the measured length is given by the formula,
Ca = LC / l ------------------- (1)
Where Ca = the correction for absolute length.
L = the measured length of a line (in m)
C = the correction to be applied to the tape.
l = the nominal length of tape (in m)
The tape length changes due to temperature while taking measurements the correction Ct is applied. It is given by the formula,
Ct = a (Tm – To) L-----------(2)
Where Ct = the correction for temperature, in m.
a = the coefficient of thermal expansion. 11 x10-6 /C.
Tm = the mean temperature during measurement
To = the temperature at which the tape is standardized
L = The area length in m.
The coefficient of Inver tape (3.6*10-7/C) is very small so it gives better results.
Tm > To Correction is positive
Tm > To Correction is negative
During measurement applied full may be either more or less at which tape or chain was Standardized due to the elastic property of material this strain will vary according to variation of applied pull. So, the correction should be applied is given by the expression.
CP = a (Pm – Po) L/ AE
Where Cp = the correction for pull in meters
P = the pull applied during measurement, in newtons (N).
Po= the pull under which the tape is standardized in newtons (N)
L = the measured length in meters.
A = the cross-sectional area of the tape, in sq.cm.
E = the modulus of elasticity of steel.
The value of E for steel may be taken as 19.3 to 20.7 x 1010 N/m2 and that for invar 13.8 to 15.2 x 1010 N/m2.
Pm > Po Correction is positive
Pm > Po Correction is negative
The correction for sag (or sag correction) is the difference in length between the arc and the subtending chord.
It is required only when the tape is suspended during measurement.
Since the effect of the set on the tapes is to make the measured length too great this correction is always subtractive. It is given by the formula,
Cs = W2L / 24P2
Cs = the sag correction for a single span, in meters.
L = the distance between supports in meters.
W= weight per meter length (Total mass of the tape in kilograms) W= mgl1
m = the mass of the tape, in kilograms per meter.
l1 = the distance between supports in meters
P = the applied pull, in newtons (N).
The pull or tension which when applied when the tape is suspended to the air, equalize the correction due to pull and sag is known as normal tension.
For one tape length, Cpull = Csag
a (Pm – Po) L/ AE = W2L / 24Pm2
Pm2 (Pm – Po) = W2AE/24
P = (0.240W √AE)/(√ Pm – Po)
The value of P is calculated by the trial and error method.
C=h^2/2L
This correction is always subtractive from the measured length.
In COMPASS SURVEY Method of traversing is used.
In TRAVESING there are numbers of connected line are used and length of them is measured with the help of tape or chain and direction is measured by angle measuring instruments.
When we use compass for measuring the angle, we call it as compass survey or traversing.
TERMINOLOGY
Imaginary semi-circle joining the earth's poles, and crossing the equator and all latitudes (baselines) at right angles. All meridians traverse in north-south direction and their ends converge at north and south poles. Meridian lines are used as one of the reference points (coordinates) with baselines in land surveying grid system to locate any point on earth. Also called longitude.
The line on a plane passing through the geographical North Pole or geographical South Pole and any point on the surface of the earth is known as true meridian. It is also called as geographical meridian. The angle between true meridian and line is known as true bearing of the line. It is also known as azimuth.
When magnetic needle is suspended freely and balance properly, unaffected by magnetic substance it indicates a direction this direction is known as magnetic meridian. The angle between magnetic meridian and line is known as magnetic bearing of the line.
ARBITRARY MERIDIAN - Sometime survey of a small area a convenient direction is assume as a meridian known as Arbitrary meridian.
(i) Whole circle bearing (WCB) The magnetic bearing of a line measured clockwise from the north pole towards the line, is known as the ‘whole circle bearing’, of that line. Such a bearing may have any value between 0 ̊ and 360 ̊ .
The whole circle bearing of a line is obtained by prismatic compass.
(ii) Quadrantal bearing (QB) The magnetic bearing of a line measured clockwise or counterclockwise from the North Pole or South Pole (whichever is nearer the line) towards the East or West, is known as the ‘quadrantal bearing’ of the line. This system consists of four quadrant) Quadrantal Bearing (QBs – NE, SE, SW and NW).
The value of a quadrantal bearing lies between 0 ̊ and 90 ̊, but the quadrants should always be mentioned.
Quadrantal bearings are obtained by the surveyor’s compass.
WCB QB QUADRANT
0 ̊-90 ̊ RB= WCB NE
90 ̊-180 ̊ RB= 180 ̊ – WCB SE
180 ̊-270 ̊ RB= WCB – 180 ̊ SW
270 ̊-360 ̊ RB= 360 ̊ – WCB NW
Fore bearing the bearing of a line measured in the direction of the progress of survey is called the ‘Fore Bearing’ (FB) of the line.
The bearing of a line measured in the direction opposite to the survey is called the ‘Back Bearing’ (BB) of the line.
For example, FB of AB = θ
BB of AB = θ1
(a) In the WCB system, the difference between the FB and BB should be exactly 180 ̊, and the negative sign when it is more than 180 ̊. Remember the following relation-
BB = FB ± 180 ̊
Use the positive sign when FB is less than 180 ̊, and the negative sign when it is more than 180 ̊.
(b) In the quadrantal bearing (i.e. reduced bearing) system, the FB and BB are numerically equal but the quadrants are just opposite. For example, if the FB of AB is N 30 ̊ E, then its BB is S 30 ̊ W.
The horizontal angle between the magnetic meridian and true meridian is known as ‘magnetic declination’.
ISOGONIC- Equal Declination
AGONIC – Zero Declination
Variation of magnetic declination
The magnetic declination at a place is not constant. It varies due to the following reasons:
(a) Secular Variation
The Earth's magnetic field is slowly changing on time scales that range from 100 years to millions of years. The meridian swings like a pendulum in one direction for about 150 years and gradually comes to a stop and then swings back in the opposite direction. It is a slow, gradual, but unexplainable shift.
(b) Annual Variation
The magnetic declination varies due to the rotation of the earth, with its axis inclined, in an elliptical path around the sun during a year. This variation is known as ‘annual variation. The amount of variation is about 1 to 2 minutes.
(c) Diurnal Variation
The magnetic declination varies due to the rotation of the earth on its own axis in 24 hours. This variation is known as ‘dirunal variation’. The amount of variation is found to be about 3 to 12 minutes.
(d) Irregular Variation
The magnetic declination is found to vary suddenly due to some natural causes, such as earthquakes, volcanic eruptions and so on. This variation is known as ‘irregular variation’.
Dip of the magnetic needle If a needle is perfectly balanced before magnetisation, But it does not remain in the balanced position after it is magnetised. This is due to the magnetic influence of the earth. The needle is found to be inclined towards the pole.
This inclination of the needle (compass needle) with respect to the horizontal (pole) is known as the ‘dip of the magnetic needle’.
DIP ZERO AT EQUATORS.
DIP IS 90 ̊ AT POLES.
It is found that the north end of the needle is deflected downwards in the northern hemisphere and that is south end is deflected downwards in the southern hemisphere. The needle is just horizontal at the equator.
SAME DIP – ISOCLINIC
ZERO DIP LINE- ACILINIC
To compensate for the effect of local attraction, the amount of error is found out and is equally distributed between the fore and back bearings of the line.
1. First Method
The interior angles of a traverse are calculated from the observed bearings. Then an angular check is applied. The sum of the interior angles should be equal to (2n – 4) x 90 ̊ (n being the number of sides of the traverse). If it is not so, the total error is equally distributed among all the angles of the traverse. Then, starting from the unaffected line, the bearings of all the lines may be corrected by using the corrected interior angles. This method is very laborious and is not generally employed.
2. Second Method
In this method, the interior angles are not calculated. From the given table, the unaffected line is first detected. Then, commencing from the unaffected line, the bearings of the other affected lines are corrected by finding the amount of correction at each station. This is an easy method, and one which is generally employed. If all the lines of a traverse are found to be affected by local attraction, the line with minimum error is identified. The FB and BB of this line are adjusted by distributing the error equally. Then, starting from this adjusted line, the fore and back bearing of other lines are corrected.
In this method, the fore and back bearings of the traverse legs are measured by prismatic compass and the sides of the traverse by chain or tape. Then the observed bearings are verified and necessary corrections for local attraction are applied. In this method, closing error may occur when the traverse is plotted. This error is adjusted graphically by using ‘Bowditch’s rule’ (which is described later on).
1. Check on angular measurements
(a) The sum of the measured interior angles should be equal to (2N – 4) x 90 ̊ where N is the number of sides of the traverse.
(b) The sum of the measured exterior angles should be equal to (2N + 4) x 90 ̊.
(c) The algebraic sum of the deflection angles should be equal to 360 ̊.
(d) Right-hand deflection is considered positive and left-hand deflection negative.
2. Check on linear measurement
(a) The lines should be measurement once each on two different days (along opposite directions). Both measurements should tally.
(b) Linear measurements should also be taken by the stadia method. The measurements by chaining and by the stadia method should tally.
In open traverse, the measurements cannot be checked directly. But some field measurements can be taken to check the accuracy of the work. The methods are discussed below.
1. Taking cut-off lines Cut-off lines are taken between some intermediate stations of the open traverse. Suppose ABCDEFG represents an open traverse. Let AD and DG be the cut-off lines. The lengths and magnetic bearings of the cut-off lines are measured accurately. After plotting the traverse, the distances and bearings are noted from the map. These distances and bearings should tally with the actual records from the field.
2. Taking an auxiliary point Suppose ABCDEF is an open traverse. A permanent point P is selected on one side of it. The magnetic bearings of this point are taken from the traverse stations A, B, C, D, etc. If the survey is carried out accurately and so is the plotting, all the measured bearings of P when plotted should meet at the point P. The permanent point P is known as the ‘auxiliary point’.
PRISMATIC COMPASS
The magnetic needle is BOARD type.
The magnetic needle is EDGE type.
The magnetic needle is attached to a graduated aluminum ring and does not rotates with line of sight.
The magnetic needle is not attached to a graduated aluminum ring and rotates with line of sight.
The graduation marked Thus
0 ̊ is at the south,
90 ̊ at the west,
180 ̊ at north and
270 ̊ at the east.
Measured WCB The graduation marked Thus
0 ̊ NORTH & SOUTH
90 ̊ at EAST & WEST
Measured QB
The graduations are engraved INVERTED since the graduated ring is read through the prism.
The graduations are engraved ERECT since, graduated ring read directly.
Reading are taken with the help of prism.
Reading are directly taken by seeing on the ring.
Sighting and reading can be done simultaneously.
Sighting and reading cannot be done simultaneously.
Instrument can also be taken in hand.
Instrument can not be used without tripod.
Eye vane consists of metal vane with large slits. No mirror.
Eye vane consists of small metal vane with small slits. A mirror is provided with the sight vane Least Count of Prismatic Compass is 30 minutes Least count of Surveyors Compass is 15 minutes.
(FIELD PROCEDURE OF OBSERVING BEARING)
1. Fixing the compass with tripod stand
2. Centering
3. Levelling
4. Adjustment of prism
5. Observation of bearing
The aim of levelling is to determine the relative height of different objects on or below the surface of earth and determine undulations of the ground surface.Level Surface
The surface which is parallel to the mean spheroidal surface of the earth is known as level surface.
The line representing the level surface is termed as level line.
The level line makes right angles to the vertical line or plumb line at any point.
Vertical Line
It is the line which is indicated by plumb at required station. So, this is also called as plumb line. It’s just decided based on the consideration of earth’s gravity.
Vertical line connects the station point to the centre of the earth.
Horizontal Line
Horizontal line is the line of sight of instrument which is tangential to the level surface and It is perpendicular the plumb line.
The surface along horizontal line of sight is called as horizontal surface.
DATUM
Datum plane is an arbitrarily assumed level surface or line with reference to which level of other line or surface are calculated.
M.S.L. is obtained by making hourly observations of the tides at any place over a period of 19 years. MSL adopted by Survey of India is now Bombay which was Karachi earlier.
B.M. is a fixed reference point of known elevation. It may be of the following types. (i) GTS Bench mark (Geodetic Triangulation Survey)
These Bench marks are established by national agency like Survey of India. They are established with highest precision. Their position and elevation above MSL are given in a special catalogue known as GTS Maps (100 km. interval).
(ii) Permanent Bench Mark- They are fixed points of reference establish with reference to GTS Bench mark (10 km. interval).
(iii) Arbitrary Bench mark-These are reference points whose elevations are arbitrarily assumed. In most of Engineering projects, the difference in elevation is more important than their reduced levels with reference to MSL as given in a special catalogue known as GTS Maps (100 Km. interval).
Height or depth of a point above or below the assumed datum is called Reduced level.
It is the line joining the intersection of the cross hair and the optical centre of the object glass and its extensions, it is also called line of sight or collimation.
The elevation of the line of sight with respect to assumed datum is known as height of Instrument (HI).
The first sight taken on a levelling staff held at a point of known elevation. B.S. enables the surveyor to obtain HI +sight i.e. Height of Instrument or line of sight.
It is the last staff reading taken from a setting of the level. It is also termed as minus sight. Fore sight is the sight taken on a levelling staff held at a point of unknown elevation to ascertain the amount by which the point is above or below the line of sight. This is also called minus sight as the foresight reading is always subtracted from height of Instrument.Change Point (CP)
The point on which both the foresight and back sight are taken during the operation of levelling is called change point.
The foresight taken on a levelling staff held at a point between two turning points, to determine the elevation of that point, is known as intermediate sight. It may be noted that for one setting of a level, there will be only one back sight and one foresight but there can be any number of intermediate sights.
The principle of levelling is to obtain horizontal line of sight with respect to which vertical distances of the points above or below this line of sight are found.
Where the difference of level is determined by only one setting of instruments is enough or only two points are required are known as simple levelling. Let instrument is setup at L1.
Elevation of point A is 100.0
RL of A is = 100 m
Height of instrument = RL of A + BACK SIGHT
HI = 100 + 1.610
HI = 101.610
After taking BS turn turn the instrument and take FS, Now
Height of instrument = RL of B + FORE SIGHT
RL of B = Height of instrument – FORE SIGHT
= 101.610 – 2.450
= 99.160
RECIPROCAL LEVELLING ELIMINATES THE ERROR DUE TO CURVATURE AND REFRACTION This is used to determine the difference of elevation of two points on opposite banks of river or deep George.
Suppose A and B are two points on the opposite banks of a river. The level is set up near A and after proper temporary adjustment, staff readings are taken at A and B. Suppose the readings are a₁ and b₁
(2) The level is shifted and set up very near B and after proper adjustment, staff reading is taken at A and B. suppose the readings are a₂ and b₂
Let h = true difference of level between A and B e = combined error due curvature, refraction and collimation (The error may be positive or negative, here we assume positive)
In the first case, In the second case, Correct staff reading at A = a₁ Correct staff reading at B = b₂ (as the level very near A) (as the level very near B) Correct staff reading at B = b₁ - e Correct staff reading at A = a₂ - e True difference between A and B, True difference of level, h = a₁ – (b₁ – e) ….. (1) h = (a₂ –e) - b₂ …. (2) (fall from B to A) From equation (1) and (2), 2h = a₁ – (b₁ – e) + (a₂ – e) - b₂ = a₁ – b₁ + e + a₂ – e - b₂ h=([(a₁ – b₁)+(a₂ – b₂)])/2 It may be observed that the error is eliminated and that the true difference is equal to the mean of two apparent differences of level between A and B. यह देखा जा सकता है कि त्रुटि समाप्त हो गई है और यह सही अंतर ए और बी के बीच के दो स्पष्ट अंतरों के माध्य के बराबर है।
Differential leveling is performed when the distance between two points is more. In this process, number of inter stations are located and instrument is shifted to each station and observed the elevation of inter station points. Finally difference between original two points is determined. Differential leveling is use when-
the points are at a great difference apart,
the difference of elevation between the points is large,
there are obstacles between the points. To find elevation of non-indivisible points. This method is called compound leveling or continuous leveling.
Fly leveling is conducted when the benchmark is very far from the work station. In such case, a temporary bench mark is located at the work station which is located based on the original benchmark. Even it is not highly precise it is used for determining approximate level.
Only back sight and fore sight readings are taken at every set up of the level and no distances are measured along the direction of leveling.
Low precision, to find/check approximate level, generally used during reconnaissance survey.
Precise leveling is similar to differential leveling but in this case higher precise is wanted. To achieve high precise, serious observation procedure is performed. The accuracy of 1 mm per 1 km is achieved.
Profile levelling or Longitudinal Levelling
Profile leveling is generally adopted to find elevation of points along a line such as for road, rails or rivers etc. In this case, readings of intermediate stations are taken and reduced level of each station is found. From this cross section of the alignment is drawn.
The fly leveling is done at the end of day's work starting point on that particular day’s known as check leveling.
(II) Indirect or Trigonometric Leveling
The process of leveling in which the elevation of point or the difference between points is measured from the observed horizontal distances and vertical angles in the field is called trigonometric leveling.
By measuring vertical angles and horizontal distance; Less precise.
(III) Stadia Levelling: Using tachometric principles. It is a modified form of trigonometric leveling in which Tachometer principle is used to determine the elevation of point. In this case the line of sight is inclined from the horizontal. It is more accurate and suitable for surveying in hilly terrains.
(IV) Barometric Levelling
Barometer is an instrument used to measure atmosphere at any altitude. So, in this method of leveling, atmospheric pressure at two different points is observed, based on which the vertical difference between two points is determined. It is a rough estimation and used rarely.
Based on atmospheric pressure difference; Using altimeter; Very rough estimation.
Height of collimation system
Rise and fall system
Height of instrument method deals with obtaining the RL of the line of collimation by adding BS reading of a point whose RL is known. The RL of line of collimation is called Height of Instrument. From this, the staff readings of all intermediate stations is subtracted to get the RL at those points.
1.Elevation of plane of collimation for the first set of the level determined by adding BACK SIGHT to R.L. of B.M.
2. The R.L. of intermediate point and first change point are then obtained by starching the staff reading taken on respective point (IS & FS) from the elation of the plane collimation. [H.I.]
3. When the instrument is shifted to the second position a new plane collimation is set up. The elevation of this plane is obtained by adding B.S. taken on the C.P. From the second position of the level to the R.L. C.P. The R.L. of successive point and second C.P. are found by subtract these staff reading from the elevation of second plane of collimation Arithmetical check Sum of B.S. – sum of F.S. = last R.L. – First R.L.
Sum of BS > Sum of FS = last point is HIGHER than the first
Sum of BS < Sum of FS = last point is LOWER than the first
This method is simple and easy.
Reduction of levels is easy.
Visualization is not necessary regarding the nature of the ground.
There is no check for intermediate sight readings
This method is generally used where a greater number of readings can be taken with a smaller number of change points for constructional work and profile levelling.
IT is a method of surveying to carrying Bench Mark from a known point to another unknown point.
In this method BS and FS reading are taken by fly levelling.
It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between there staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point.
Arithmetic check
Sum of B.S. – sum of F. S. = sum of rise – sum of fall = last R. L. – first R.L.
This method is complicated and is not easy to carry out.
Reduction of levels takes more time.
Visualization is necessary regarding the nature of the ground.
Complete check is there for all readings.
This method is preferable for check levelling where number of change points are more. HEIGHT OF INSTRUMENT RISE AND FALL METHOD
It is rapid as it involves few Calculation.
It is laborious involving several calculations.
There is no check on the RL of the intermediate sight.
There is a check on the RL of the intermediate points.
Errors in the intermediate RLs cannot be detected.
Errors in the intermediate RLs can be detected as all the points are correlated.
There are two checks on the accuracy of RL calculation.
There are three checks on the accuracy of RL calculation.
This system is suitable for longitudinal levelling where number of intermediate sights.
This system is suitable for fly there are a levelling where there are no intermediate sights.
Dumpy level is the most commonly used instrument in levelling. A dumpy level, also known as an automatic level or builder’s level. Setting up dumpy level
Find a benchmark location near the spot you want to measure. A benchmark location is a spot that you already know the height of thanks to previous land surveys.
Set your tripod up near the spot you want to measure.
Connect your device to the tripod and position it over 2 levelling screws.
Level the device by adjusting the 2 leveling screws. Look for a traditional bubble level located somewhere on your device.
Turn your telescope 90 degrees and adjust the third leveling screw.
Check your level’s calibration by turning it 180 degrees. Eye piece
Eye piece is used by the observer’s eye to view the distant object. It contains magnifying glass which magnify the observing image and also the cross hairs of diaphragm.
Objective lens
Objective lens are provided at the other end of the telescope. The objective lens consists of two parts, the front part consists convex type lens and the back part consists concave lens.
Diaphragm
Diaphragm is provided in front of the eye piece. It contains cross hairs made of dark metal which are arranged in perfect perpendicular positions. These cross hairs are used by the eye piece to bisect the objective through objective lens.
Focusing screw
Focusing screw is used to adjust the focus if cross hairs and the image clarity. The magnification of eye piece is managed by this focusing screw.
The procedure of dumpy level surveying starts with some temporary adjustments which are-
Setting up of instrument
Leveling up
Focusing
Setting up of Dumpy Level
The instrument is fixed to the tripod stand using clamp screws. Spread the tripod legs and position the instrument at convenient height. Firstly, fix the two legs in the ground at a point and cantering of bubble in the bubble tubes is done by adjusting third leg.
Leveling up
The leveling up of an instrument is done using foot screws or leveling screws. In this case, the telescope is arranged parallel to the any two leveling screws and the bubble in the tube is cantered by turning both the screws either inwards or outwards.
Focusing Your Level
Adjust the eyepiece until you can see the device’s crosshairs. Place a sheet of paper or a similar object directly in front of your device’s lens to occupy its entire field of vision. Then, turn the eyepiece’s focusing knob until you can clearly see the dumpy level’s crosshairs.
When finished, your crosshairs should appear dark, sharp, and easily noticeable.There are following types of Errors in Levelling
Instrumental Errors
Personal Errors
Errors due to Natural Causes
Instrumental Errors
Error due to imperfect adjustment
Error due to sluggish Bubble
Error due to defective staff
Error due to defective Tripod
Error due to faulty focussing tube
Personal Errors
Error due to bubble out of canter
Error due to imperfect focusing
Error due to non-verticality of staff
Error due to telescope not open fully
Error due to wrong Sighting
Errors due to Natural Causes
Error due to Curvature & Refraction
Error due to temperature variations
Error due to sun or wind
Error due to shimmering effect
Error due to tripod settlement
Curvature correction for long distances, the curvature of earth affects the staff reading. Level line is a curved line, parallel to the MSL level surface, but the horizontal lines goes straight. This means that the vertical distance of that target from the level line is going to be larger than the distance which we calculate from the horizontal line. The amount of correction depends upon the magnitude of the horizontal distance between the target and the instrument station. The vertical distance between the line of sight and level line a a particular place is called curvature correction. P Distance (d) Q OP2+PQ2= OQ2
R2+d2 =(R+Cc)2
R2+d2 = R2+Cc2+2RCc
d2 = Cc2+2RCc
d2 = Cc (Cc+2R)
as R>>>C
d2 = Cc (2R)
Cc=d^2/2R
Where R = 6370 km
Cc = - 0.0785d2
d is in km and curvature correction is always negative.
For Ture staff reading True Reading = Observed Staff reading – 0.0785d2
With increase in altitude density of air decrease since air is denser near the earth the ray of light from staff to instrument travel from a lighter medium to denser medium and thus it bends towards normal.
For average atmospheric condition the curved path of refracted ray may assumed to be arc of circle of radius 7R, R is radius of earth.
Cr = C_c/7
Cr = (0.0785d^2)/7
Cr = 0.0112d2
Combined Correction is = Cc + Cr
C = – 0.0785d2 + 0.0112d2
C = - 0.0673d2 Visible horizon distance
Let AB = D = visible horizon distance in kilometres
Considering curvature and refraction corrections,
h = 0.0673D²
D = √(h/(0.0673))
Dip of horizon
The angle between the horizontal line and the tangent line is known as the dip of the horizon. AB = D = tangent of earth at A
BD = horizontal line perpendicular to OB
θ = dip of horizon
Dip(θ) = (arc CA)/(radius of earth)
θ=D/R
It is equal to the subtended by the arc AC at the centre of the earth
Sensitiveness of the bubble
The term sensitiveness in the context of a bubble means the effect caused by the deviation of the bubble per division of the graduation of the bubble tube.
Sensitiveness is expressed in term of the radius of the curvature of the upper surface of the tube or by an angle through which the axis is tilted for the deflection of one division of the graduation.
Liquid used – Sprits, Alcohols Determining sensitiveness
consider Fig. Suppose the level was set up at O at a distance D from the staff at P. The staff reading is taken with the bubble at extreme right end (i.e. at E). Say it is PA. Another staff reading taken with the bubble at the extreme left end (i.e. at E₁). Let it is be PB
Let, D = distance between level and staff
S = intercept between the upper and lower sights,
n = number of divisions through the bubble is reflected
R = radius of curvature of the tube
θ = angle subtended by arc EE₁, and
d = length of the division of the graduation, expressed in the same unit as D and S.
θ = nd/R=S/D
Therefore, R = (nd* D)/S
Let θ’ = angular value for one division in radians
θ^'=θ/n*S/D*1/n radians
θ^'=S/Dn*206 265 seconds (1 radian = 206265 secs.)
θ/n*l/R*S/nD
Total division = n length of one division = l Total Length = nl
Sensitivity Can be increased by
Increase the length of tube
Increase diameter of tube
Decrease viscosity of tube
Decrease roughness of inner wall.
.
It is the line joining of equal elevation it is known as contours.OBJECT OF SURVEYING
THE aim of surveying is to prepare a map to show the relative position of objects on surface of the earth.
Primary Classification
1. Plane Surveying2. Geodetic Surveying
In Plane survey the curvature of earth is not taken in to consideration. The lines joining any two points is considered as to be straight, and the triangle formed by any three points is also assumed as plane triangle. The area considered below the 250 km2.
In geodetic survey the curvature of earth is taken into consideration in order to have high degree of precision. The lines joining any two points is considered as a curved line. The triangle formed by any three points is also assumed as spherical lines. The area considered above the 250 km2.
Secondary Classification
Classification Based on Place of Survey
LAND SURVEY
Survey being done on land. Land surveying can be sub-divided into following categories-Topographical Surveys
Cadastral Surveys
City Surveys
Engineering Surveys
HYDROGRAPHIC SURVEY
Survey of water bodies, to established shore lines, Navigation possibilities.UNDERGROUND SURVEY
Survey required for construction of tunnel, mines.AERIAL SURVEY
It is carried out by taking photographs with cameras fitted on airplanes, helicopter. This is used to make a large-scale map.Classification Based on PURPOSE or Objective of Survey
On the basis of object of survey, the classification can be as given below.(1) Control survey
(2) Hand survey
(3) Topographic survey
(4) Engineering survey
(a) Reconnaissance survey
(b) Preliminary survey
(c) Location survey
(5) Route survey
(6) Construction survey
(7) Astronomic survey
(8) Mine survey
GEOLOGICAL SURVEY
Information about both surface and sub surfaces acquired for assessing for different type of reserve like the minerals, rocks and also folds, faults and helps in determining the type of foundation required and which soil treatment is required.
GEOGRAPHICAL SURVEY
This is done for depicting the land use efficiency, irrigation intensity, surface drainage, slope profiles, x contours.
ENGINEERING SURVEY
It is done to prepare detailed drawing of projects involving roads, railways, dams, water supply design, reservoirs, bridges etc.
1) Reconnaissance survey
To explore site conditions and availability of infrastructures.
2) Preliminary survey
To collect adequate data to prepare plan / map of area to be used for planning and design.
3) Location survey
To set out work on the ground for actual construction / execution of the project.
CADRASTRAL SURVEY
To established boundaries of properties for legal purpose for fields, estates and houses, etc.
DEFENCE SURVEY
Surveys done for military purpose, provide strategic information for deciding the future course of action.
TOPOGRAPHICAL SURVEY
It is done to determine the natural features of a country.
ARCHEOLOGICAL SURVEY
It is done to gather the information about ancient monuments, towns, villages, kingdoms.
CITY SUREVY
It is carried out to locate the premises, streets water supplies sanitary systems, etc.
CONTROL SURVEYING
To establish horizontal and vertical positions of control points.
ASTRONOMIC SURVEYS
To determine the latitude, longitude (of the observation station) and azimuth (of a line through observation station) from astronomical observation.
ROUTE SURVEY
To plan, design, and laying out of route such as highways, railways, canals, pipelines, and other linear projects.
CONSTRUCTION SURVEYS
Surveys which are required for establishment of points, lines, grades, and for staking out engineering works (after the plans have been prepared and the structural design has been done).
Classification Based on INSTRUMENTS USED
CHAIN SURVEY
Chain surveying is the simplest method of surveying in which the linear measurements are directly taken in the field and the angular measurements are not taken. This type of surveying is used over small and levelled area.
TRAVERSE / COMPASS SURVEY
Here both linear and angular measurements are made, former with made with tape or chain later with compass. This type e of survey useful for large project like dam or reservoirs.
LEVELLING SURVEY
Here elevation of different points are determined. Graduated staff and level, dumpy or automatic level is used.
TACHEOMETRY
In this method of surveying in which the horizontal and vertical distances of relative points are determined with the graduated staff with a transit telescope fitted with anallatic lenses.
PLANE TABLE SURVEY
Here observation and plotting is done simultaneously in field it is mainly used for small and medium scale mapping where great accuracy is not required.
TRIANGULATION SURVEY
I this surveying whole area is divided into a network of triangles to determine distances and relative positions of points spread over an area, by measuring the length of one side of each triangle and deducing its angles and length of other two sides by observation from this baseline.
EDM SURVEY
EMD refers Electronic Device Measurement and in this method, distance are measured electronically using wave propagation, reflection and subsequent reception of the reflected wave.
TOTAL STATION SURVEY
It the combination of transit theodolite with electronic distance meter (EDM).It is also integrated with microprocessor, electronic data collector and storage system. It is used to measure sloping distance of object to the instrument, horizontal angles and vertical angles.
Data collected from total station can be downloaded into computer/laptops for further processing of information.
SATELLITE SURVEY
Here information about the land is determined by using satellite-based navigation system and GPS.
PRINCIPLE OF SURVEY
Work from “Whole to part"
According to the first principle the whole area is first enclosed by main station and survey line. The area is then divided into a number of parts by forming well-conditioned Triangle.The purpose of this process of working is to prevent accumulation of errors during this procedure if there is any error in the measurement of any side of a triangle then it will not affect the whole work. The error can always be detected and eliminated.
According to second principle the new station should always be fixed by at least two measurements linear or angular from fix reference points.
Method of linear Measurement-
By passing or steeping- The walking step of man considered as 2.5 feet 80 cm.Passometer- It count the number of steps automatically.
Perambulator- it is a wheel fitted with fork and handle. Wheel is graduated and shows a distance per revolution.
Angular measurement refers to magnetic bearing or horizontal angle taken by a PRISMATIC COMPASS or THEODOLITE.
CHAIN SURVEY
It is the branch of Surveying in which the distance is measured with a Chain and tape, this operation is called chaining.The principle of chain surveying is TRIANGULATION this means that the area to be surveyed is divided into a number of small triangles that should be well conditioned.
Preferably, all the sides of a triangle should be nearly equal having each angle nearly 60° to ensure minimum distortion due to errors in measurement of sides and plotting.
Generally, such an ideal condition is practically not possible always due to configuration of the terrain and, therefore, an attempt should be made to have well-conditioned triangles in which no angle is smaller than 30° and no angle is greater than 120°.
The arrangement of triangles to be adopted in the field depends on the shape, topography, and the natural or artificial obstacles met with.
Chain survey recommended when
The ground surface is more or less level.
The area is to be surveyed.
A small-scale map is to be prepared.
The formation of a well-conditioned triangle is easy.
Chain surveying is unsuitable when
First, the area is crowded with many details.
The area consists of two many undulations.
The area is very large.
The formation of the well-conditioned triangle becomes difficult due to obstacles.
SURVEY STATION
MAIN SURVEY STATION
Station taken along the boundary of an area as a controlling point is known as the main survey station. the line joining the main station is called the main survey line.
Main Survey Station- A, B, C, D
Tie Station- E, F
Main Survey Line- AB, BC, CD, DA
Tie Survey Line- DE, FB
Base Line- AC
Check Line- BH, BG
TIE STATION OR SUBSIDIARY STATIONS
The station which is on the main survey line or any other survey line is known as a subsidiary station.
BASE LINE
The line on which the framework of a survey is built is known as a baseline the long survey line which is run through the middle of the area to be surveyed.
CHECK LINE
The line is run to check the accuracy of the traverse consisting of a framework of triangles. It joins the apex point of a triangle to some fixed point on its base is known as the check line. it is taken to check the accuracy of the triangle sometimes this line help to locate interior details.
OFFSETS
The lateral measurement taken from an object to the chain line is called offset. Offsets are taken to locate the object with reference to the chain line.
Perpendicular offsets
When the lateral measurement is taken perpendicular to the chain line they are known as perpendicular offset.
oblique offsets
oblique you upset is taken when the object is at a long distance from the chain line it is not possible to set up a right angle due to some difficulties.
CHAINAGE
Chainage is the horizontal distance as measured along with a combination of curves and straight lines (curvilinear) between two points. Chainage It is the distance of a well-defined point from the starting point. In chain surveying, it is normally referred to as the distance of the foot of the offset from the starting point on the chain line.
The operation of measuring the distance is termed as chaining/taping. The Point of Beginning of the line is usually denoted as 00+00 (The Starting Point) with stationing at least every 100 feet along the line denoted as 1+00 (for 100 feet)…2+00 (for 200 feet)…3+00 (for 300 feet), etc. where the first number represents the number of 100-foot stations and the digits after the (+) sign represents any remaining portion less than 100 feet.
For example 31+57.95 would represent 3,157.95 feet.
TYPE OF CHAIN
Metric chains
Metric chains are the most commonly used chain in India. These types of chains come in many lengths such as 5, 10, 20 and 30 meters.
The most commonly used is 20m and 30m chain.
The brass ring is placed at every meter.
The total length of the chain is marked on the brass handle at the ends.
20 meter chain 100 link @ 0.2m, Tallies after 10 links(2m). [0.2*10=2m]
30 meter chain 150 link @ 0.2m, Tallies after 25 links(5m). [0.2*25=5m]
Revenue Chain
The standard size of this type of chain is 33ft. The number of links is 16, each link being 2(1/16) ft. This chain is commonly used in the cadastral survey.
33ft = 10.06 m link @ (33/16 =62.87).
Gunter’s chain or surveyor’s chain
Gunter chain comes in standard 66ft. This chain consists of 100links, each link being 0.66ft or 7.92inches.
The length 66ft is selected because it is convenient in land measurements.
1 mile = 8 Furlongs = 80 Gunter chains
1 Acre = 10 square Gunter’s chains
10 Gunter chains = 1 Furlong
Engineer’s chain
This chain comes in 100ft length. It consists of 100 links each link being 1ft long.
At every 10 links, a brass ring or tags are provided for an indication of 10 links.
Readings are taken in feet and decimals.
Testing and Adjustment of Chain
As the chain is a metal made, it may undergo many changes due to temperature effect or human error and etc. So, for all lengths of the chain, a tolerance is given,
10m chain = + or – 3mm
20m chain = + or – 5mm
30m chain = + or – 8mm
Errors in chain Surveying
Personal Errors
Wrong reading, wrong recording, reading from the wrong end of the chain, etc., are personal errors. These errors are serious errors and cannot be detected easily. Care should be taken to avoid such errors. Compensating Errors
The compensating errors are those which are liable to occur in both direction i.e (positive and negative) and finally hence tend to compensate are known as compensating error.
Hence, they are likely to get compensated when a large number of readings are taken.
They are directly proportional to √L
In chaining, these may be caused by the following: -
The incorrect holding of the chain: - The follower may not bring his handle of the chain to the arrow, but may hold it to one or the other side of the arrow.
Fractional parts of the chain or tape may not be correct if the total length of the chain is adjusted by insertion or removal of a few connection rings from one portion of the chain, or tape is not calibrated uniformly throughout its length. Graduations in the tape may not be exactly the same throughout.
During stepping operation crude method of plumbing (such as dropping of stone from the end of the chain) is adopted.
When chain angles are set out with a chain that is not uniformly adjusted or with a combination of chain and tape.
Cumulative Errors
The cumulative errors are those which occur in the same direction and tend to add up or accumulate i.e. either to make the apparent measurement always too long or too short.
In each reading, the error may be small, but when a large number of measurements are made, they may be considerable, since the error is always on one side.
Examples of such errors are:
Bad ranging
Bad straightening
Temperature variation
Variation in applied pull
Non-horizontal it
They are directly proportional to L.
Positive errors
(making the measured lengths more than the actual) are caused by the following:-
The length of the chain or tape is shorter than the standard, because of bending of links, removal of too many links in adjusting the length, ‘knots’ in the connecting links, cloggings of rings with clay, temperature lower than that at which the tape was calibrated, shrinkage of tape when becoming wet. The slope correction is not applied to the length measured along the sloping ground.
The sag correction is not applied when the tape or the chain is suspended in the air.
Measurements are made along the faulty alignment.
The tape is in suspension or in high winds.
Negative errors
(making the measured lengths less than the actual) maybe caused because the length of the tape or chain may be greater than the standard because of the wear or flattening of the connecting rings,
opening of ring joints,
temperature higher than the one at which it was calibrated.
Applied more pull than the standard pull.
TAPES
Tapes are used in surveying to take linear measurements. They are available in different lengths and can be made of different materials. There are 5 types of tapes available in surveying for linear measurements and they are as follows
Linen Tape.
Metallic Tape.
Steel Tape.
Synthetic Tape.
Invar Tape.
Linen tape, also known as cloth tape is a varnished strip made of closely woven linen and it is varnished to resist moisture.
15mm wide. Available in 10m,15m.
Used for taking offset for ordinary work.
Metallic Tape, when linen tape is reinforced with brass and copper wire to make it durable, then it is called metallic tape.
available in different lengths of 10m, 15m, 20m, 30m, and 50m.
These are used for survey works such as topographical survey works.
Steel tape, is made of steel or stainless steel.
It consists of a steel strip of 6mm to 16mm wide.
Synthetic tapes are made of glass fibers coated with PVC.
These are light in weight and flexible.
Synthetic tapes may stretch when subjected to tension. Hence, these are not suitable for accurate surveying works.
Invar tapes,
36% of nickel and 64% of steel.
6mm wide strip and is available in different lengths of 30m, 50m, 100m.
The coefficient of thermal expansion of the invar alloy is very low. It is not affected by changes in temperature. Hence, these tapes are used for high precision works
Accuracy Sequence of tapes
Invar Tape > Steel Tape > Metallic Tape > Linen Tape
OTHER EQUIPMENT
Ranging rod is a surveying instrument used for making a LINE STRAIGHT. And also marking the position of stations, and for sightings of those stations. Made with the well-seasoned wood such as teak, pine or deodar, and GI pipes. Diameter- 25mm Length- 2 meter Alternate BLACK & WHITE Strip of length 0.2 meter. The lower end is pointed and provided with the iron shore.
Offset Rods
These are similar to ranging rods except at the top where a stout open ring recessed hook is provided, It is also provided with two short narrow vertical slots at right angles to each other, passing through the center of the section, at about eye level. It is mainly used to align the offset line and measuring the short offsets. With the help of the hook provided at the top of the rod, the chain can be pulled or pushed through the hedges or other obstructions, if required. Offsets may also be made in the field with the help of a cross-staff or optical square.
Arrows
When the length of the line to be measured is more than a chain length, there is a need to mark the end of the chain length. Arrows are used for this purpose. Arrows are made up of 4 mm diameter steel wire with one end sharpened and another end bent into a loop. The diameter of the loop is 50mm, Total length is 400mm.
Pegs
Wooden pegs are used in measuring the length of a line to mark the endpoints of the line. The pegs are made of hardwood of 25 mm × 25 mm section, 150 mm long with one end sharp. When driven in the ground to mark station points, the project about 40 mm. Clinometer is an instrument used for measuring angles of slope (or tilt).
Cross-Staff
It is essentially an instrument used for setting out right angles. In its simplest form, it is known as Open Cross-Staff (a). It consists of two pairs of vertical slits providing two lines of sight mutually at right angles. Another modified form of the cross-staff is known as French Cross-Staff (b). This consists of an octagonal brass tube with slits on all eight sides. This has a distinct advantage over the open cross-staff as with it even lines at 45° can be set out from the chain line. The latest modified cross-staff is the Adjustable Cross-Staff (c). It consists of two cylinders of equal diameter placed one above the other. The upper cylinder can be rotated over the lower one graduated in degrees and its subdivisions. The upper cylinder carries the Vernier and the slits to provide a line of sight. Thus, it may be used to take offsets and to set out any desired angle from the chain line.
Optical Square
Optical Square It is more accurate than the cross-staff.
It is a small and compact hand instrument and works on the principle of reflection. Generally, it is a 5 cm in diameter and 1.25 cm deep.
used for locating objects situated at larger distances. a metal cover to protect it from dust, moisture, etc.
It consists of a horizontal mirror (H) and an index mirror (l) placed at an angle of 45 degrees to each other.
The mirror H is half-silvered and the upper half is plain while the mirror I is fully silvered. There are three openings a, b and c on the sides.
Let AB is the chain line and it is required to locate an object O during the process of surveying. The optical square is held in such a manner that a ray of light from object O passes through slot c, strikes the mirror, gets reflected and strikes the silvered portion of the mirror H. After being reflected from H, the ray passes through the pinhole and becomes visible to the eye. The observer looking through the hole a can directly see the ranging rod at B through the un-silvered portion of the mirror H and he image of the ranging rod placed at O. Thus, when both the ranging rods coincide, the line OD becomes perpendicular to the chain line. If they do not coincide, the optical square has to move back and forth to get the correct position of D.
Ranging
The term ranging is used to establish a set of intermediate points on a straight line whose two ends have already been fixed on the ground, whereas Chaining means measuring the length of a straight line with chain or tape. Chaining a long line necessarily involves ranging. There are two methods of ranging-
When an intermediate ranging rod is fixed in a straight line by direct observation from an end station, the process is known as direct ranging.
When end station is not visible due to there being high ground between them intermediate ranging rod fixed on the line in an indirect way this method is known as indirect ranging or (reciprocal ranging)
Taping on a Sloping or Uneven Ground
If the slope of the ground is more than 3° or 1 in 20, there is a considerable difference between the slope distance and the horizontal distance. One of the following procedures may be adopted to determine the horizontal equivalent of the measured slope distance in such cases:
1. Direct method 2. Indirect method.
Direct method: In the direct method, also known as the stepping method, the horizontal equivalent of the slope distance is directly measured, as shown in Fig. It is more convenient to measure downhill than to measure uphill and, therefore, uphill measurement should be avoided as far as possible. In the case of downhill measurements, the horizontal distance is measured in steps as shown in Fig. 3.8a using the procedure explained for taping on the flat ground. For taping uphill as shown in Fig. 3.8b, the rear end of the tape is held at A' above A and the other end at B on the line AE, and the measurement proceeds upwards in a similar manner as above.
Indirect Method- When the slope is steeper than the steeping method is not suitable. In such a case, the horizontal distance is measured by the following method.
Measuring the slope with Clinometer.
By Applying for Hypotenuse allowance
By knowing the difference of level between two points.
1. In this method, the angle QPR can be measured by a clinometer or on the vertical circle of the transit.
Then. PQ= PQ cos Hypotenuse allowance is one of the indirect methods of ranging on uneven or sloping ground. In this method, a correction is applied at every chain length and at every point where the slope changes. This facilitates locating or surveying the intermediate points. Let α = the angle of slope of the ground.
AD = AB = 1 Chain = 100 links.
Then AC = (100 sec) α links and
BC = AC – AB
BC = (100 Sec α) – (100) links.
BC = 100 (Sec α- 1) links.
The amount 100 (sec α – 1) is known as a hypotenuse allowance.
3.Knowing the difference in the level
Suppose, A, B, C, and D are different points on sloping ground. The difference of level between these points is determined by a leveling instrument. Let the respective differences be h1, h2, and h3. Then the sloping distances AB, BC, and CD are measured. Let the distances L1, L2, and L3 respectively (Fig).
Obstacles in Chaining of a Line | Land Survey | Surveying
The three main obstacles in the chaining of a line are of the following types:
1. Chaining Free, Vision Obstructed
2. Chaining Obstructed, Vision Free
3. Chaining and Vision Both Obstructed.
1. Chaining Free, Vision Obstructed:
In this type of obstacle, the ends of the lines are not intervisible e.g. rising ground, hill or jungle intervening.
(i) Both ends may be visible from any intermediate point lying on the line such as in the case of a hill. The obstacle of this kind may easily be crossed over by reciprocal ranging and length measured by the stepping method of chaining. (ii) Both ends may not be visible from any intermediate point such as in the case of a jungle. The obstacle of this kind may be crossed over by the “Random line method”. 2. Chaining Obstructed, Vision Free
The typical obstacle of this type is a sheet of water, the width of which in the direction of measurement exceeds the length of the chain or tape. The problem consists in finding the distance between convenient points on the chain line on either side of the obstacle. (a) When the obstacle can be chained around, e.g. a pond, a thorny hedge, etc. (b) When the obstacle cannot be chained around e.g. a river. 3. Chaining and Vision Both Obstructed A building is a typical example of this class of obstacles. The problem, in this case, consists both in prolonging the line beyond the obstacle and finding the distance across it.
SCALE OF MAP
It is not always representing the actual distance of an object on the drawing.
The scale is the ratio of the distance of two-point on a sheet or map to the same distance of two-point in the ground.
Representative Fraction (RF)=(Distance beteen two point on sheet)/(Same distance on the ground)
Small Scale- 1 cm = 100 m
Medium Scale- 1 cm = 50 m
Large Scale- 1 cm = 10 m
Smallest scale is that whose denominator is larger value.
The scales are classified into four categories-
Plain Scale
Diagonal Scale
Scale of chords
Vernier Scale
Plain Scale
Plain Scale is one on which it is possible to measure two dimensions only. For example, measurements such as units and lengths, meters and decimeters, etc.
Diagonal Scale
On the diagonal scale, it is possible to measure three dimensions such as meters, decimeters and centimeters, units, tens and hundreds; yards, feet, and inches, etc. A short length is divided into a number of parts using the principle of the similar triangle in which sides are proportional. 1-1 represent 1/10 PQ
2-2 represent 2/10 PQ
9-9 represent 9/10 PQ
Scale of Chords
The scale of chords is used to measure an angle and is marked on either on a rectangular protractor or an ordinary boxwood scale.
Vernier Scale
A device used for measuring the fractional part of one of the smallest divisions of a graduated scale.
It usually consists of a small auxiliary scale that slides alongside the main scale.
Least count of the Vernier = the difference between smallest division on the main scale(S) and smallest division on the Vernier scale(v).
Least Count = S-v
Direct Vernier
It is directly calibrated in same direction of the main scale.
The Direct Vernier scale divisions are shorter than the Main Scale divisions. If it is required to read 1/nth part of the smallest division on the main scale, (n-1) main divisions are taken and divided into n equal divisions on the Vernier scale.
nv=(n-1) d
n = the number of divisions on the vernier.
v = the value of the smallest division on the Vernier scale.
d= the value of the smallest division on the Main scale.
Retrograde Vernier
It is directly calibrated in the opposite direction of the main scale.
In this type of the vernier, (n+1) divisions of the main scale are taken and divided into n divisions on the vernier scale.
nv=(n+1) d
(i) Vernier divisions are longer than the main scale divisions,
(ii) The graduations of the main scale are marked in the direction opposite to that of the vernier scale-one from right to left and the other from left to right.
The only advantage of a retrograde vernier is that the graduations are bigger than those of a direct vernier. But as it has to be read in the opposite direction, which is rather difficult, it is not commonly used.
Extended vernier nv=(2n-1) d
Double Vernier
With a simple vernier, readings can be taken in one direction only, but a double vernier is required when the graduations on the main scale are marked in both discussions from the common zero, such as at Abney’s level. In a double vernier, two simple Verniers are placed end to end forming one scale with the zero in the center. One is used for readings in the clockwise direction and the other for the readings in the anticlockwise direction. In the case of a vernier attached to the vertical circle of a transit theodolite which is divided into the quadrants, two sets of graduations are marked on a single vernier instead of providing a double vernier. In reading this vernier, only that set is used which increases in the same direction as the graduations on the quadrant which is being read. There are some other special forms of vernier such as an extended vernier used on the astronomical sextant, and the double-folded vernier used in compasses, etc.
SHRINKAGE RATIO
Shrinkage Ratio=(Shrunk length)/(Actual length)= (Shrunk Scale)/(Origional Scale)
Shrinkage ratio / factor is < 1 or equal to 1.
Correction due to incorrect Tape Length
True length of chain=(True length±Errors)/(Standard length of chain)*Measured Length(ML)
Corrections in Chain Surveying
Correction for Absolute Length
If the actual tape length is not equal to standard value then the correction should be applied.
The correction for the measured length is given by the formula,
Ca = LC / l ------------------- (1)
Where Ca = the correction for absolute length.
L = the measured length of a line (in m)
C = the correction to be applied to the tape.
l = the nominal length of tape (in m)
Correction for Temperature
The tape length changes due to temperature while taking measurements the correction Ct is applied. It is given by the formula,
Ct = a (Tm – To) L-----------(2)
Where Ct = the correction for temperature, in m.
a = the coefficient of thermal expansion. 11 x10-6 /C.
Tm = the mean temperature during measurement
To = the temperature at which the tape is standardized
L = The area length in m.
The coefficient of Inver tape (3.6*10-7/C) is very small so it gives better results.
Tm > To Correction is positive
Tm > To Correction is negative
Correction for Pull
During measurement applied full may be either more or less at which tape or chain was Standardized due to the elastic property of material this strain will vary according to variation of applied pull. So, the correction should be applied is given by the expression.
CP = a (Pm – Po) L/ AE
Where Cp = the correction for pull in meters
P = the pull applied during measurement, in newtons (N).
Po= the pull under which the tape is standardized in newtons (N)
L = the measured length in meters.
A = the cross-sectional area of the tape, in sq.cm.
E = the modulus of elasticity of steel.
The value of E for steel may be taken as 19.3 to 20.7 x 1010 N/m2 and that for invar 13.8 to 15.2 x 1010 N/m2.
Pm > Po Correction is positive
Pm > Po Correction is negative
Correction for Sag (subtractive)
The correction for sag (or sag correction) is the difference in length between the arc and the subtending chord.
It is required only when the tape is suspended during measurement.
Since the effect of the set on the tapes is to make the measured length too great this correction is always subtractive. It is given by the formula,
Cs = W2L / 24P2
Cs = the sag correction for a single span, in meters.
L = the distance between supports in meters.
W= weight per meter length (Total mass of the tape in kilograms) W= mgl1
m = the mass of the tape, in kilograms per meter.
l1 = the distance between supports in meters
P = the applied pull, in newtons (N).
Normal-Tension
The pull or tension which when applied when the tape is suspended to the air, equalize the correction due to pull and sag is known as normal tension.
For one tape length, Cpull = Csag
a (Pm – Po) L/ AE = W2L / 24Pm2
Pm2 (Pm – Po) = W2AE/24
P = (0.240W √AE)/(√ Pm – Po)
The value of P is calculated by the trial and error method.
Correction for Slope
C=h^2/2L
This correction is always subtractive from the measured length.
Compass SURVEY
In COMPASS SURVEY Method of traversing is used.
In TRAVESING there are numbers of connected line are used and length of them is measured with the help of tape or chain and direction is measured by angle measuring instruments.
When we use compass for measuring the angle, we call it as compass survey or traversing.
TERMINOLOGY
MERIDIAN
Imaginary semi-circle joining the earth's poles, and crossing the equator and all latitudes (baselines) at right angles. All meridians traverse in north-south direction and their ends converge at north and south poles. Meridian lines are used as one of the reference points (coordinates) with baselines in land surveying grid system to locate any point on earth. Also called longitude.
TRUE MERIDIAN
The line on a plane passing through the geographical North Pole or geographical South Pole and any point on the surface of the earth is known as true meridian. It is also called as geographical meridian. The angle between true meridian and line is known as true bearing of the line. It is also known as azimuth.
MAGNETIC MERIDIAN
When magnetic needle is suspended freely and balance properly, unaffected by magnetic substance it indicates a direction this direction is known as magnetic meridian. The angle between magnetic meridian and line is known as magnetic bearing of the line.
ARBITRARY MERIDIAN - Sometime survey of a small area a convenient direction is assume as a meridian known as Arbitrary meridian.
GRID MERIDIAN
- Sometimes, for preparing a map some state agencies assume several lines parallel to the true meridian for a particular zone. These lines are termed as ‘grid lines’ and the central line the ‘grid meridian’. The bearing of a line with respect to the grid meridian is known as the ‘grid bearing’ of the line. Designation of magnetic bearing Magnetic bearings are designated by two systems-(i) Whole circle bearing (WCB) The magnetic bearing of a line measured clockwise from the north pole towards the line, is known as the ‘whole circle bearing’, of that line. Such a bearing may have any value between 0 ̊ and 360 ̊ .
The whole circle bearing of a line is obtained by prismatic compass.
(ii) Quadrantal bearing (QB) The magnetic bearing of a line measured clockwise or counterclockwise from the North Pole or South Pole (whichever is nearer the line) towards the East or West, is known as the ‘quadrantal bearing’ of the line. This system consists of four quadrant) Quadrantal Bearing (QBs – NE, SE, SW and NW).
The value of a quadrantal bearing lies between 0 ̊ and 90 ̊, but the quadrants should always be mentioned.
Quadrantal bearings are obtained by the surveyor’s compass.
WCB QB QUADRANT
0 ̊-90 ̊ RB= WCB NE
90 ̊-180 ̊ RB= 180 ̊ – WCB SE
180 ̊-270 ̊ RB= WCB – 180 ̊ SW
270 ̊-360 ̊ RB= 360 ̊ – WCB NW
FORE BEARING
Fore bearing the bearing of a line measured in the direction of the progress of survey is called the ‘Fore Bearing’ (FB) of the line.
BACK BEARING
The bearing of a line measured in the direction opposite to the survey is called the ‘Back Bearing’ (BB) of the line.
For example, FB of AB = θ
BB of AB = θ1
(a) In the WCB system, the difference between the FB and BB should be exactly 180 ̊, and the negative sign when it is more than 180 ̊. Remember the following relation-
BB = FB ± 180 ̊
Use the positive sign when FB is less than 180 ̊, and the negative sign when it is more than 180 ̊.
(b) In the quadrantal bearing (i.e. reduced bearing) system, the FB and BB are numerically equal but the quadrants are just opposite. For example, if the FB of AB is N 30 ̊ E, then its BB is S 30 ̊ W.
Magnetic declination
The horizontal angle between the magnetic meridian and true meridian is known as ‘magnetic declination’.
ISOGONIC- Equal Declination
AGONIC – Zero Declination
Variation of magnetic declination
The magnetic declination at a place is not constant. It varies due to the following reasons:
(a) Secular Variation
The Earth's magnetic field is slowly changing on time scales that range from 100 years to millions of years. The meridian swings like a pendulum in one direction for about 150 years and gradually comes to a stop and then swings back in the opposite direction. It is a slow, gradual, but unexplainable shift.
(b) Annual Variation
The magnetic declination varies due to the rotation of the earth, with its axis inclined, in an elliptical path around the sun during a year. This variation is known as ‘annual variation. The amount of variation is about 1 to 2 minutes.
(c) Diurnal Variation
The magnetic declination varies due to the rotation of the earth on its own axis in 24 hours. This variation is known as ‘dirunal variation’. The amount of variation is found to be about 3 to 12 minutes.
(d) Irregular Variation
The magnetic declination is found to vary suddenly due to some natural causes, such as earthquakes, volcanic eruptions and so on. This variation is known as ‘irregular variation’.
DIP OF MAGNETIC NEEDLE
Dip of the magnetic needle If a needle is perfectly balanced before magnetisation, But it does not remain in the balanced position after it is magnetised. This is due to the magnetic influence of the earth. The needle is found to be inclined towards the pole.
This inclination of the needle (compass needle) with respect to the horizontal (pole) is known as the ‘dip of the magnetic needle’.
DIP ZERO AT EQUATORS.
DIP IS 90 ̊ AT POLES.
It is found that the north end of the needle is deflected downwards in the northern hemisphere and that is south end is deflected downwards in the southern hemisphere. The needle is just horizontal at the equator.
SAME DIP – ISOCLINIC
ZERO DIP LINE- ACILINIC
LOCAL ATTRACTION
A magnetic needle indicates the north direction when freely suspended or pivoted. But if the needle comes near some magnetic substances, such as iron ore, steel structures, electric cables conveying current; etc. it is found to be deflected from its true direction, and does not show the actual north. This disturbing influence of magnetic substances is known as ‘local attraction’. If the difference of the fore and back bearings of the line is exactly 180 ̊, then there is no local attraction.To compensate for the effect of local attraction, the amount of error is found out and is equally distributed between the fore and back bearings of the line.
METHOD OF APPLICATION OF CORRECTION
1. First Method
The interior angles of a traverse are calculated from the observed bearings. Then an angular check is applied. The sum of the interior angles should be equal to (2n – 4) x 90 ̊ (n being the number of sides of the traverse). If it is not so, the total error is equally distributed among all the angles of the traverse. Then, starting from the unaffected line, the bearings of all the lines may be corrected by using the corrected interior angles. This method is very laborious and is not generally employed.
2. Second Method
In this method, the interior angles are not calculated. From the given table, the unaffected line is first detected. Then, commencing from the unaffected line, the bearings of the other affected lines are corrected by finding the amount of correction at each station. This is an easy method, and one which is generally employed. If all the lines of a traverse are found to be affected by local attraction, the line with minimum error is identified. The FB and BB of this line are adjusted by distributing the error equally. Then, starting from this adjusted line, the fore and back bearing of other lines are corrected.
Compass traversing
In this method, the fore and back bearings of the traverse legs are measured by prismatic compass and the sides of the traverse by chain or tape. Then the observed bearings are verified and necessary corrections for local attraction are applied. In this method, closing error may occur when the traverse is plotted. This error is adjusted graphically by using ‘Bowditch’s rule’ (which is described later on).
CHECK ON CLOSED TRAVERSE
1. Check on angular measurements
(a) The sum of the measured interior angles should be equal to (2N – 4) x 90 ̊ where N is the number of sides of the traverse.
(b) The sum of the measured exterior angles should be equal to (2N + 4) x 90 ̊.
(c) The algebraic sum of the deflection angles should be equal to 360 ̊.
(d) Right-hand deflection is considered positive and left-hand deflection negative.
2. Check on linear measurement
(a) The lines should be measurement once each on two different days (along opposite directions). Both measurements should tally.
(b) Linear measurements should also be taken by the stadia method. The measurements by chaining and by the stadia method should tally.
CHECK ON OPEN TRAVERSE
In open traverse, the measurements cannot be checked directly. But some field measurements can be taken to check the accuracy of the work. The methods are discussed below.
1. Taking cut-off lines Cut-off lines are taken between some intermediate stations of the open traverse. Suppose ABCDEFG represents an open traverse. Let AD and DG be the cut-off lines. The lengths and magnetic bearings of the cut-off lines are measured accurately. After plotting the traverse, the distances and bearings are noted from the map. These distances and bearings should tally with the actual records from the field.
2. Taking an auxiliary point Suppose ABCDEF is an open traverse. A permanent point P is selected on one side of it. The magnetic bearings of this point are taken from the traverse stations A, B, C, D, etc. If the survey is carried out accurately and so is the plotting, all the measured bearings of P when plotted should meet at the point P. The permanent point P is known as the ‘auxiliary point’.
TYPES OF COMPASS
PRISMATIC COMPASS
SURVEYORS COMPASS
The magnetic needle is BOARD type.The magnetic needle is EDGE type.
The magnetic needle is attached to a graduated aluminum ring and does not rotates with line of sight.
The magnetic needle is not attached to a graduated aluminum ring and rotates with line of sight.
The graduation marked Thus
0 ̊ is at the south,
90 ̊ at the west,
180 ̊ at north and
270 ̊ at the east.
Measured WCB The graduation marked Thus
0 ̊ NORTH & SOUTH
90 ̊ at EAST & WEST
Measured QB
The graduations are engraved INVERTED since the graduated ring is read through the prism.
The graduations are engraved ERECT since, graduated ring read directly.
Reading are taken with the help of prism.
Reading are directly taken by seeing on the ring.
Sighting and reading can be done simultaneously.
Sighting and reading cannot be done simultaneously.
Instrument can also be taken in hand.
Instrument can not be used without tripod.
Eye vane consists of metal vane with large slits. No mirror.
Eye vane consists of small metal vane with small slits. A mirror is provided with the sight vane Least Count of Prismatic Compass is 30 minutes Least count of Surveyors Compass is 15 minutes.
TEMPORARY ADJUSTMENT OF PRISMATIC COMPASS
(FIELD PROCEDURE OF OBSERVING BEARING)
1. Fixing the compass with tripod stand
2. Centering
3. Levelling
4. Adjustment of prism
5. Observation of bearing
LEVELLING
LEVELLING
The aim of levelling is to determine the relative height of different objects on or below the surface of earth and determine undulations of the ground surface.
Level Surface
The surface which is parallel to the mean spheroidal surface of the earth is known as level surface.
The line representing the level surface is termed as level line.
The level line makes right angles to the vertical line or plumb line at any point.
Vertical Line
It is the line which is indicated by plumb at required station. So, this is also called as plumb line. It’s just decided based on the consideration of earth’s gravity.
Vertical line connects the station point to the centre of the earth.
Horizontal Line
Horizontal line is the line of sight of instrument which is tangential to the level surface and It is perpendicular the plumb line.
The surface along horizontal line of sight is called as horizontal surface.
DATUM
Datum plane is an arbitrarily assumed level surface or line with reference to which level of other line or surface are calculated.
Mean Sea Level (M.S.L.)
M.S.L. is obtained by making hourly observations of the tides at any place over a period of 19 years. MSL adopted by Survey of India is now Bombay which was Karachi earlier.
BENCH MARK – (BM)
B.M. is a fixed reference point of known elevation. It may be of the following types. (i) GTS Bench mark (Geodetic Triangulation Survey)
These Bench marks are established by national agency like Survey of India. They are established with highest precision. Their position and elevation above MSL are given in a special catalogue known as GTS Maps (100 km. interval).
(ii) Permanent Bench Mark- They are fixed points of reference establish with reference to GTS Bench mark (10 km. interval).
(iii) Arbitrary Bench mark-These are reference points whose elevations are arbitrarily assumed. In most of Engineering projects, the difference in elevation is more important than their reduced levels with reference to MSL as given in a special catalogue known as GTS Maps (100 Km. interval).
REDUCED LEVEL (RL)
Height or depth of a point above or below the assumed datum is called Reduced level.
Line of Collimation
It is the line joining the intersection of the cross hair and the optical centre of the object glass and its extensions, it is also called line of sight or collimation.
Height of Instrument (HI)
The elevation of the line of sight with respect to assumed datum is known as height of Instrument (HI).
Back Sight (B.S.)
The first sight taken on a levelling staff held at a point of known elevation. B.S. enables the surveyor to obtain HI +sight i.e. Height of Instrument or line of sight.
Fore Sight (F.S.)
It is the last staff reading taken from a setting of the level. It is also termed as minus sight. Fore sight is the sight taken on a levelling staff held at a point of unknown elevation to ascertain the amount by which the point is above or below the line of sight. This is also called minus sight as the foresight reading is always subtracted from height of Instrument.
Change Point (CP)
The point on which both the foresight and back sight are taken during the operation of levelling is called change point.
Intermediate Sight
(IS)The foresight taken on a levelling staff held at a point between two turning points, to determine the elevation of that point, is known as intermediate sight. It may be noted that for one setting of a level, there will be only one back sight and one foresight but there can be any number of intermediate sights.
Principle of Levelling
The principle of levelling is to obtain horizontal line of sight with respect to which vertical distances of the points above or below this line of sight are found.
Simple Levelling
Where the difference of level is determined by only one setting of instruments is enough or only two points are required are known as simple levelling. Let instrument is setup at L1.
Elevation of point A is 100.0
RL of A is = 100 m
Height of instrument = RL of A + BACK SIGHT
HI = 100 + 1.610
HI = 101.610
After taking BS turn turn the instrument and take FS, Now
Height of instrument = RL of B + FORE SIGHT
RL of B = Height of instrument – FORE SIGHT
= 101.610 – 2.450
= 99.160
RECIROCAL LEVELLING
Reciprocal levelling is used to determine the correct difference of elevations of the two points which are quite far apart and when it is not possible to set up the instrument mid-way between the two points to balance the fore sight and back sight.RECIPROCAL LEVELLING ELIMINATES THE ERROR DUE TO CURVATURE AND REFRACTION This is used to determine the difference of elevation of two points on opposite banks of river or deep George.
Suppose A and B are two points on the opposite banks of a river. The level is set up near A and after proper temporary adjustment, staff readings are taken at A and B. Suppose the readings are a₁ and b₁
(2) The level is shifted and set up very near B and after proper adjustment, staff reading is taken at A and B. suppose the readings are a₂ and b₂
Let h = true difference of level between A and B e = combined error due curvature, refraction and collimation (The error may be positive or negative, here we assume positive)
In the first case, In the second case, Correct staff reading at A = a₁ Correct staff reading at B = b₂ (as the level very near A) (as the level very near B) Correct staff reading at B = b₁ - e Correct staff reading at A = a₂ - e True difference between A and B, True difference of level, h = a₁ – (b₁ – e) ….. (1) h = (a₂ –e) - b₂ …. (2) (fall from B to A) From equation (1) and (2), 2h = a₁ – (b₁ – e) + (a₂ – e) - b₂ = a₁ – b₁ + e + a₂ – e - b₂ h=([(a₁ – b₁)+(a₂ – b₂)])/2 It may be observed that the error is eliminated and that the true difference is equal to the mean of two apparent differences of level between A and B. यह देखा जा सकता है कि त्रुटि समाप्त हो गई है और यह सही अंतर ए और बी के बीच के दो स्पष्ट अंतरों के माध्य के बराबर है।
Differential levelling
Differential leveling is performed when the distance between two points is more. In this process, number of inter stations are located and instrument is shifted to each station and observed the elevation of inter station points. Finally difference between original two points is determined. Differential leveling is use when-
the points are at a great difference apart,
the difference of elevation between the points is large,
there are obstacles between the points. To find elevation of non-indivisible points. This method is called compound leveling or continuous leveling.
Fly levelling
When differential leveling is done in order to connect a bench mark to the starting point of the alignment of any project, it is called fly leveling.Fly leveling is conducted when the benchmark is very far from the work station. In such case, a temporary bench mark is located at the work station which is located based on the original benchmark. Even it is not highly precise it is used for determining approximate level.
Only back sight and fore sight readings are taken at every set up of the level and no distances are measured along the direction of leveling.
Low precision, to find/check approximate level, generally used during reconnaissance survey.
Precise levelling
Precise leveling is similar to differential leveling but in this case higher precise is wanted. To achieve high precise, serious observation procedure is performed. The accuracy of 1 mm per 1 km is achieved.
Profile levelling or Longitudinal Levelling
Profile leveling is generally adopted to find elevation of points along a line such as for road, rails or rivers etc. In this case, readings of intermediate stations are taken and reduced level of each station is found. From this cross section of the alignment is drawn.
Check leveling
The fly leveling is done at the end of day's work starting point on that particular day’s known as check leveling.
(II) Indirect or Trigonometric Leveling
The process of leveling in which the elevation of point or the difference between points is measured from the observed horizontal distances and vertical angles in the field is called trigonometric leveling.
By measuring vertical angles and horizontal distance; Less precise.
(III) Stadia Levelling: Using tachometric principles. It is a modified form of trigonometric leveling in which Tachometer principle is used to determine the elevation of point. In this case the line of sight is inclined from the horizontal. It is more accurate and suitable for surveying in hilly terrains.
(IV) Barometric Levelling
Barometer is an instrument used to measure atmosphere at any altitude. So, in this method of leveling, atmospheric pressure at two different points is observed, based on which the vertical difference between two points is determined. It is a rough estimation and used rarely.
Based on atmospheric pressure difference; Using altimeter; Very rough estimation.
There are two Methods of Levelling-
Height of collimation system
Rise and fall system
1. Height of instrument method
Height of instrument method deals with obtaining the RL of the line of collimation by adding BS reading of a point whose RL is known. The RL of line of collimation is called Height of Instrument. From this, the staff readings of all intermediate stations is subtracted to get the RL at those points.
1.Elevation of plane of collimation for the first set of the level determined by adding BACK SIGHT to R.L. of B.M.
2. The R.L. of intermediate point and first change point are then obtained by starching the staff reading taken on respective point (IS & FS) from the elation of the plane collimation. [H.I.]
3. When the instrument is shifted to the second position a new plane collimation is set up. The elevation of this plane is obtained by adding B.S. taken on the C.P. From the second position of the level to the R.L. C.P. The R.L. of successive point and second C.P. are found by subtract these staff reading from the elevation of second plane of collimation Arithmetical check Sum of B.S. – sum of F.S. = last R.L. – First R.L.
Sum of BS > Sum of FS = last point is HIGHER than the first
Sum of BS < Sum of FS = last point is LOWER than the first
This method is simple and easy.
Reduction of levels is easy.
Visualization is not necessary regarding the nature of the ground.
There is no check for intermediate sight readings
This method is generally used where a greater number of readings can be taken with a smaller number of change points for constructional work and profile levelling.
2.Rise and Fall method
IT is a method of surveying to carrying Bench Mark from a known point to another unknown point.
In this method BS and FS reading are taken by fly levelling.
It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between there staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point.
Arithmetic check
Sum of B.S. – sum of F. S. = sum of rise – sum of fall = last R. L. – first R.L.
This method is complicated and is not easy to carry out.
Reduction of levels takes more time.
Visualization is necessary regarding the nature of the ground.
Complete check is there for all readings.
This method is preferable for check levelling where number of change points are more. HEIGHT OF INSTRUMENT RISE AND FALL METHOD
It is rapid as it involves few Calculation.
It is laborious involving several calculations.
There is no check on the RL of the intermediate sight.
There is a check on the RL of the intermediate points.
Errors in the intermediate RLs cannot be detected.
Errors in the intermediate RLs can be detected as all the points are correlated.
There are two checks on the accuracy of RL calculation.
There are three checks on the accuracy of RL calculation.
This system is suitable for longitudinal levelling where number of intermediate sights.
This system is suitable for fly there are a levelling where there are no intermediate sights.
Types of Levels Used in Levelling
1. Dumpy LevelDumpy level is the most commonly used instrument in levelling. A dumpy level, also known as an automatic level or builder’s level. Setting up dumpy level
Find a benchmark location near the spot you want to measure. A benchmark location is a spot that you already know the height of thanks to previous land surveys.
Set your tripod up near the spot you want to measure.
Connect your device to the tripod and position it over 2 levelling screws.
Level the device by adjusting the 2 leveling screws. Look for a traditional bubble level located somewhere on your device.
Turn your telescope 90 degrees and adjust the third leveling screw.
Check your level’s calibration by turning it 180 degrees. Eye piece
Eye piece is used by the observer’s eye to view the distant object. It contains magnifying glass which magnify the observing image and also the cross hairs of diaphragm.
Objective lens
Objective lens are provided at the other end of the telescope. The objective lens consists of two parts, the front part consists convex type lens and the back part consists concave lens.
Diaphragm
Diaphragm is provided in front of the eye piece. It contains cross hairs made of dark metal which are arranged in perfect perpendicular positions. These cross hairs are used by the eye piece to bisect the objective through objective lens.
Focusing screw
Focusing screw is used to adjust the focus if cross hairs and the image clarity. The magnification of eye piece is managed by this focusing screw.
The procedure of dumpy level surveying starts with some temporary adjustments which are-
Setting up of instrument
Leveling up
Focusing
Setting up of Dumpy Level
The instrument is fixed to the tripod stand using clamp screws. Spread the tripod legs and position the instrument at convenient height. Firstly, fix the two legs in the ground at a point and cantering of bubble in the bubble tubes is done by adjusting third leg.
Leveling up
The leveling up of an instrument is done using foot screws or leveling screws. In this case, the telescope is arranged parallel to the any two leveling screws and the bubble in the tube is cantered by turning both the screws either inwards or outwards.
Focusing Your Level
Adjust the eyepiece until you can see the device’s crosshairs. Place a sheet of paper or a similar object directly in front of your device’s lens to occupy its entire field of vision. Then, turn the eyepiece’s focusing knob until you can clearly see the dumpy level’s crosshairs.
When finished, your crosshairs should appear dark, sharp, and easily noticeable.
There are following types of Errors in Levelling
Instrumental ErrorsPersonal Errors
Errors due to Natural Causes
Instrumental Errors
Error due to imperfect adjustment
Error due to sluggish Bubble
Error due to defective staff
Error due to defective Tripod
Error due to faulty focussing tube
Personal Errors
Error due to bubble out of canter
Error due to imperfect focusing
Error due to non-verticality of staff
Error due to telescope not open fully
Error due to wrong Sighting
Errors due to Natural Causes
Error due to Curvature & Refraction
Error due to temperature variations
Error due to sun or wind
Error due to shimmering effect
Error due to tripod settlement
CORRECTION DUE TO CURVATURE & REFRACTION
Curvature correction
Curvature correction for long distances, the curvature of earth affects the staff reading. Level line is a curved line, parallel to the MSL level surface, but the horizontal lines goes straight. This means that the vertical distance of that target from the level line is going to be larger than the distance which we calculate from the horizontal line. The amount of correction depends upon the magnitude of the horizontal distance between the target and the instrument station. The vertical distance between the line of sight and level line a a particular place is called curvature correction. P Distance (d) Q OP2+PQ2= OQ2
R2+d2 =(R+Cc)2
R2+d2 = R2+Cc2+2RCc
d2 = Cc2+2RCc
d2 = Cc (Cc+2R)
as R>>>C
d2 = Cc (2R)
Cc=d^2/2R
Where R = 6370 km
Cc = - 0.0785d2
d is in km and curvature correction is always negative.
For Ture staff reading True Reading = Observed Staff reading – 0.0785d2
Effect of Refraction
With increase in altitude density of air decrease since air is denser near the earth the ray of light from staff to instrument travel from a lighter medium to denser medium and thus it bends towards normal.
For average atmospheric condition the curved path of refracted ray may assumed to be arc of circle of radius 7R, R is radius of earth.
Cr = C_c/7
Cr = (0.0785d^2)/7
Cr = 0.0112d2
Combined Correction is = Cc + Cr
C = – 0.0785d2 + 0.0112d2
C = - 0.0673d2 Visible horizon distance
Let AB = D = visible horizon distance in kilometres
Considering curvature and refraction corrections,
h = 0.0673D²
D = √(h/(0.0673))
Dip of horizon
The angle between the horizontal line and the tangent line is known as the dip of the horizon. AB = D = tangent of earth at A
BD = horizontal line perpendicular to OB
θ = dip of horizon
Dip(θ) = (arc CA)/(radius of earth)
θ=D/R
It is equal to the subtended by the arc AC at the centre of the earth
Sensitiveness of the bubble
The term sensitiveness in the context of a bubble means the effect caused by the deviation of the bubble per division of the graduation of the bubble tube.
Sensitiveness is expressed in term of the radius of the curvature of the upper surface of the tube or by an angle through which the axis is tilted for the deflection of one division of the graduation.
Liquid used – Sprits, Alcohols Determining sensitiveness
consider Fig. Suppose the level was set up at O at a distance D from the staff at P. The staff reading is taken with the bubble at extreme right end (i.e. at E). Say it is PA. Another staff reading taken with the bubble at the extreme left end (i.e. at E₁). Let it is be PB
Let, D = distance between level and staff
S = intercept between the upper and lower sights,
n = number of divisions through the bubble is reflected
R = radius of curvature of the tube
θ = angle subtended by arc EE₁, and
d = length of the division of the graduation, expressed in the same unit as D and S.
θ = nd/R=S/D
Therefore, R = (nd* D)/S
Let θ’ = angular value for one division in radians
θ^'=θ/n*S/D*1/n radians
θ^'=S/Dn*206 265 seconds (1 radian = 206265 secs.)
θ/n*l/R*S/nD
Total division = n length of one division = l Total Length = nl
Sensitivity Can be increased by
Increase the length of tube
Increase diameter of tube
Decrease viscosity of tube
Decrease roughness of inner wall.
COUNTOURS
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Contour Interval (C.I.)
Vertical distance between any two consecutive contours is known as the contour interval.
It depends upon the scale of the map, nature of the ground and availability of the fund time.
It depends on scale of map.
C.I. = 1/(Scale of map)
For topographic survey CI = 20/(Number of cm per km)
Horizontal Equivalent (H.E.)
It is the shortest horizontal distance between the two consecutive contours.
It depends on slope.
For C.I. = 10 m
H.E. = 5 * C.I.
= 5*10 =50m
Contour Gradient (C.G.)
Gradient represents the ascending or descending slope of the terrain between two consecutive contour lines. The slope or gradient is usually stated in the format 1 in S, where 1 represents the vertical component of the slope and S its corresponding horizontal component measured in the same unit. Contour Gradient (C.G.)= (C.I.)/(H.E.)
STEEP SLOPE = CLOSED CONTOUR
GENTLE SLOPE = FAR AWAY COUNTORS
UNIFORM SLOPE = EQUALLY SPACED CONTOURS
PLANE TABLE SURVEY
Plane Table Surveying
Plane table surveying is a graphical method of surveying in which the field works and the plotting is done simultaneously.
It is particularly use for small areas mapping.
Plane table surveying is used for locating the field computation of area of field.
Merits
1. It is most suitable for preparing small scale map or surveying small area.
2. It is most rapid method.
3. Field book is not necessary.
4. No great skill is required for satisfactory map.
5. It is particularly suitable for magnetic area where prismatic compass is not reliable.
6. Contour and irregular object may be represented accurately.
7. It is less costly.
Demerits
1. Plane Table Essentially a tropical instrument.
2. It is not suitable to work in wet climate.
3. There are several accessories to be carried out and therefore they are likely to be lost.
4. It is not suitable for accurate work.
Plane Table essentially consists of a Drawing board mounted on tripod stand and Alidade.
1. Drawing board mounted on tripod
A sheet of drawing paper, called plane table sheet is fastened to the board.
Board is made up of well-seasoned wood such as teak of size 40x30 cm to 75x60 cm.
2. Alidade
The alidade is a ruler with a sight line attached and is used on the plane table for bisecting the object, drawing rays, direction lines, etc.
It consists of a metal (brass or gunmetal) or boxwood straightedge or ruler of about 45 cm long.
The bevelled edge is called the “ruling edge” or the “working edge” or the “fiducial edge.”
The alidade fitted with sight vanes at both the ends,
One of the sight vanes is provided with a narrow rectangular slit. While other is provided with a central vertical hair or wire.
The alidade is equipped with a telescope,
If the alidade is telescopic, the telescope is provided with a vertical circle, and a level tube is fitted with cross-hairs.
3. Trough Compass
It consists of an 80 to 150 mm long and 30 mm wide box carrying a freely suspended needle at its centre.
At the ends of the needle graduations are marked on the box to indicate zero to five degrees on either side of the centre. The box is provided with glass top to prevent oscillation of the needle by wind.
When needle is centred (reading 0–0), the line of needle is parallel to the edge of the box. Hence marking on the edges in this state indicates magnetic north–south direction.
4.Plumbing Fork and Plumb Bob
Plumbing fork is a U-shaped metal frame with an upper horizontal arm and a lower inclined arm.
The upper arm is provided with a pointer at the end while the lower arm is provided with a hook to suspend plumb bob.
When the plumbing fork is kept on the plane table the vertical line (line of plumb bob) passes through the pointed edge of upper arm.
The plumb bob helps in transferring the ground point to the drawing sheet and vice versa also.
PRINCIPAL OF PLANE TABLE SURVEY
(1) To distinguish clearly, small letters like p, q, r, etc. should be used on the paper to represent the corresponding points P, Q, R, etc. on the ground.
(2) While taking the sights, the table should be clamped in position, and the alidade must be cantered on the plotted station point on the paper or drawing sheet.
(3) The table must only be turned for orientation, and when it is correctly oriented, it should be again clamped in position.
The setting up of the table includes three operations,
(1) Levelling the table;
(2) Orienting the table; and
(3) Cantering the table, i.e., (the point on the paper representing the station being occupied should be vertically above the point on the ground).
Levelling - The plane table should be set up at a convenient height (nearly a meter) by spreading the legs to keep the table approximately levelled OR by using of levelling screws (if provided) or by tilting the board by hand if the instrument has a ball and socket arrangement.
Orientation - may be defined as the operation keeping the table at each of the successive stations parallel to the position occupied by the table at the first station.
Centring - is an essential step in plain table survey. The centring means to set the table vertically on the ground point.
Methods of plane table surveying
1. RadiationIn this method of plain table survey, the plane table is set up at only one station, and various points are located by radiating (drawing) a ray from the instrument station to each of the points, and plotting to scale along the ray the distance measured from the station to the point is sighted. The radiation method is suitable for surveys of small areas which are likely to be commanded from a single station.
2. Intersection or triangulation method of plane table surveying
This method is widely employed for plotting the details on the maps. It can also be used for plotting the position of points to be used at subsequent stations. The various points can be located by the intersection of rays drawn from two different stations (A and B) forming a baseline. The only linear measurement required is that of the baseline AB on the ground.
The method may also be employed for locating the distant and inaccessible objects, the rivers, in a survey of the hilly areas (where distances cannot be measured easily), and for checking the remote objects.
3. Traversing method of plane table survey
Traversing by the plane table is similar to compass or theodolite traversing. The method, therefore, can be used for laying down the survey lines of a closed or unclosed traverse. The details may be plotted in the usual way by taking offsets. The plane table traversing is also useful for the survey of roads, rivers, etc.
4. Resection
The method of resection is employed for the location of station points only.
After establishing the stations, the details are located by either radiation or intersection.
The main feature of this method is that the point plotted on the sheet is the station occupied by the table. Only one linear measurement is required as in the intersection method of plane tabling. In this method the plotted position of objects is known and the plotted position of station is obtained.
If a, b and c are the plotted positions of objects A, B and C respectively, to locate instrument station P on the paper, the orientation of table is achieved with the help of a, b, c and then resectors Aa, Bb, Cc are drawn to get the ‘p’ , the plotted position of P. Hence in the resection method major work is to ensure suitable orientation by any one of the methods. The following methods are employed in the method of resection-
(a) Resection after Orientation by Compass
(b) Resection after Orientation by Back Sighting
(c) by solving Two Point Problem
(d) by solving Three Point Problem.
(a) Resection after Orientation by Compass
Let a and b be the plotted positions of A and B of two well defined points in the field. Keeping the through compass along north direction marked on the drawing sheet table is oriented on station P, the position of which is to be found on paper. The resectors Aa and Bb [Fig.] are drawn to locate ‘p’ the plotted position of station point P. This method gives satisfactory results, if the area is not influenced by local attractions. It is used for small scale mapping only.
(b) Resection after Orientation by Back Sighting Figure shows the scheme of resection after orientation by back sighting. From station A, the position of B is plotted as ‘b’ and ray has been taken to station P as ap′. Then plane table is set at P and oriented by back sighting A, line AP is not measured but the position of P is obtained on the paper by taking resection Bb.
(c) Resection after Solving Two Point Problem The problem of finding plotted position of the station point occupied by the plane table with the help of plotted positions of two well defined points is known as solving two-point problem. Figure shows the scheme of solving this. Let A and B be two well defined points like lightening conductor or spire of church, the plotted positions a and b already known. Now the problem is to orient the table at P so that by resection its plotted position p can be obtained.
The following steps may be followed to solve this problem:
(i) Select a suitable point Q near P such that the angles PAQ and PBQ are not accute.
(ii) Roughly orient the table at Q and draw the resectors Aa and Bb to get the point ‘q’.
(iii) Draw the ray qp and locate p1 with estimated distance QP.
(iv) Shift the plane table to P and orient the table by back sighting to Q.
(v) Draw the resector Aa to get ‘p’.
(vi) Draw the ray pB. Let it intersect line bq at b1.
(vii) The points b and b1 are not coinciding due to the angular error in the orientation of table. The angle bab, is the angular error in orientation. To correct it, * Fix a ranging rod at R along ab,
* Unclamp the table and rotate it till line ab sights ranging rod at R. Then clamp the table. This gives the correct orientation of the table which was used in plotting the points A and B. (viii) The resectors Aa and Bb are drawn to get the correct plotted position ‘p’ of the station P.
(d) Resection after Solving Three Point Problem
Locating the plotted position of a station point using observations to three well defined points whose plotted positions are known, is called solving three point problem. Let A, B, C be three well defined objects on the field whose plotted positions a, b and c are known. Now the problem is to locate plotted position of the station point P. Any one of the following methods can be used. (i) Mechanical (Tracing paper) method,
(ii) Graphical method, or
(iii) Trial and error method (Lehman’s method).
(i) Mechanical Method
This method is known as tracing paper method since it needs a tracing paper. The method involved the following steps [Ref. Fig. ]
* Set the table over station P and by observation approximately orient the table.
* Fix the tracing paper on the plane table and select P approximately, say as p′. From p′, draw p′ A, p′ B and p′ C. These lines may not pass through the plotted positions a, b and c since the orientation is not exact.
* Loosen the tracing paper and rotate it so that the rays pass through respective points a, b and c. Now prick the point p′ to get the plotted position ‘p’ of the station P.
* Keep the alidade along pa and sight A. Then clamp the table. This is correct orientation.
Check the orientation by observing along pb and pc. (ii) Graphical Method
The following two graphical methods are available to solve three point problem:
* Bessel’s solution
* Method of perpendiculars.
Bessels Solution: It involves the following steps:
1. Keep the bevelled edge of alidade along ba and sight object at A. Clamp the table and draw bc′ along the line bc [Fig. 14.14 (a)]. 2. Keep bevelled edge of alidade along ab, unclamp the table and sight B. Clamp the table.
Draw line ac intersecting bc′ at d [Fig. 14.14(b)].
3. Keep the alidade along dc and bisect C. Clamp the table [Fig. 14.14(c)]. This gives the
correct orientation.
4. Draw resectors to get ‘p’.
(iii)Trial and Error Method
This method is also known as ‘triangle of error method’ and ‘Lehman’s Method’. It involves the following steps:
1. Set the table over point P and orient the table approximately, just by observation.
2. Draw the rays aA, bB and cC [Fig. 14.16]. If the orientation was perfect, the three rays would have intersected at a single point, i.e. at point ‘p’. Otherwise a triangle of error is formed. 3. To eliminate the triangle of error an approximate position, ray p′, is selected near the triangle
of error. Then keeping alidade along p′a object A is sighted and the table is clamped. Draw the resectors cC and bB to check the orientation. 4. Above step is repeated till triangle of error is eliminated.
Lehman presented the following guidelines to select ‘p′’ so that triangle of error is eliminated quickly.
Rule 1: The distance of point sought ‘p’ is in the same proportion from the corresponding rays as the distance of those from the plane table station.
Rule 2: The point sought ‘p’ is on the same side of all the three resectors. Defining the triangle ABC on the field as great triangle and the circle passing through them as great circle, from the above two rules of Lehman, the following sub-rules may be drawn [Ref. Fig. 14.17].
* If ‘P’ lies within the great triangle, the point ‘p’ is within the triangle of error (p1 in the Fig. 14.17).
* If the plane table station P lies outside the great triangle the point sought ‘p’ is outside the triangle of errors (p2).
* If the ‘P’ is on the great circle, the correct solution is impossible (p3 and p4).
* If ‘P’ is outside the great circle, ‘p’ is nearer to the intersection of rays to the nearest two points (P5).
* If point P is outside the great circle and the two rays drawn are parallel to each other the point sought is outside the parallel lines and on the same side of the three rays (P6). -----------
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