October 2019 ~ Civil is King

CHAIN SURVEYING

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 which 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, 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

1. The ground surface is more or less level.

2. Area is to be surveyed.

3. A small-scale map is to be prepared.

4. The formation of well-conditioned triangle is easy.

Chain surveying is unsuitable when

1. First the area is crowded with many details.

2. The area consists of two many undulation.

3. Area is very large.

4. Formation of well-conditioned triangle become difficult due to obstacles.

SURVEY STATION

MAIN SURVEY STATION

Station taken along the boundary of an area as controlling point are known as main survey station. the line joining the main station are called 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

Station which are on main survey line or any other survey line are known as subsidiary station.

BASE LINE

The line on which the framework of a survey is built is known as baseline the long survey line which is run through the middle of the area to be surveyed.

CHECK LINE

Line is run to check 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 check line. it is taken to check the accuracy of triangle sometime 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 object with reference to the chain line.

1. Perpendicular offsets

When the lateral measurement is taken perpendicular to the chain line they are known as perpendicular offset.

2. oblique offsets

oblique you upset are taken when the object is at 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 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 represent any remaining portion less than 100 feet.

For example: 31+57.95 would represent 3,157.95 feet.

TYPE OF CHAIN

1. 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.

Most commonly used is 20m and 30m chain.

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]

2. Revenue Chain

The standard size of this type of chain is 33ft. The number of links are 16, each link being 2(1/16) ft. This chain is commonly used in cadastral survey.

33ft = 10.06 m link @ (33/16 =62.87).

3. 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. 1 mile = 8 Furlongs = 80 Gunter chains

2. 1 Acre = 10 square Gunter’s chains

3. 10 Gunter chains = 1 Furlong

4. 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 indication of 10 links.

Readings are taken in feet and decimal.

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 chain a tolerance is given,

10m chain = + or – 3mm

20m chain = + or – 5mm

30m chain = + or – 8mm

Errors in chain Surveying

1. Personal Errors

Wrong reading, wrong recording, reading from wrong end of chain etc., are personal errors. These errors are serious errors and cannot be detected easily. Care should be taken to avoid such errors.

2. 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 large number of readings are taken.

They are directly proportional to √L

In chaining, these may be caused by the following: -

· 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 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 tape may not be exactly same throughout.

· During stepping operation crude method of plumbing (such as dropping of stone from the end of chain) is adopted.

· When chain angles are set out with a chain which is not uniformly adjusted or with a combination of chain and tape.

3. 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 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.

4. 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.

5. Negative errors

(making the measured lengths less than the actual) may be 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

1. Linen Tape.

2. Metallic Tape.

3. Steel Tape.

4. Synthetic Tape.

5. 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 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 SSC

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.

Lower end is pointed and provided with 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 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 cross-staff or optical square.

Arrows

When the length of the line to be measured is more than a chain length, there is 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.

Diameter of loop is- 50mm, Total length is 400mm.

Pegs

Wooden pegs are used in measuring a length of a line to mark the end points of the line.

The pegs are made of hard wood of 25 mm × 25 mm section, 150 mm long with one end sharp.

When driven in ground to mark station points, they 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 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 horizontal mirror (H) and index mirror (l) placed at an angle of 45 degree 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 pin hole 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 the 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 intermediate ranging rod are fixed on a straight line by direct observation from 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 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 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 similar manner as above.

Indirect Method- When the slope is more steep than steeping method is not suitable. In such case horizontal distance is measured by following method.

1. By Measuring the slope with Clinometer.

2. By Applying Hypotenuse allowance

3. By knowing the difference of level between two points.

1.In this method the angle PQR 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 in 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 hypotenuse allowance.

3.Knowing the difference of level

Suppose, A, B, C and D are different points on sloping ground. The difference of level between these points is determined by a levelling 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 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 obstacles, 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 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 “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 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 drawing.
Scale is the ratio of distance of two point on a sheet or map to the same distance of two point in 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-

1. Plain Scale

2. Diagonal Scale

3. Scale of chords

4. Vernier Scale

1. 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.

2. Diagonal Scale

On 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 number of parts using the principle of 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

3. Scale of Chords

Scale of chords is used to measure an angle and is marked on either on rectangular protractor or an ordinary box wood scale.

4. 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 which slides alongside of 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 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 opposite direction of 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 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 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 main scale are marked in both discussions from the common zero, such as in Abney’s level.

In a double vernier, two simple Vernier’s 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 / factor is < 1 or equal to 1.

Correction due to incorrect Tape Length

Corrections in Chain Surveying

1. 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,

C_a = LC / l ------------------- (1)

Where C_a = 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 a tape (in m)

2. Correction for Temperature

The tape length is changes due to temperature, while taking measurements the correction C_t is applied. It is given by the formula,

C_t = a (T_m – T_o) L-----------(2)

Where C_t = the correction for temperature, in m.
            a = the coefficient of thermal expansion. 11 x10^-6 /C.
          T_m = the mean temperature during measurement
          T_o = the temperature at which the tape is standardized
           L = the measure length in m.

The coefficient of Inver tape (3.6*10^-7/C) is very small so it give better results.

T_m > T_o Correction is positive

T_m > T_o Correction is negative

3. Correction for Pull

During measurement applied full maybe either more or less at which tape or chain was Standardized due to 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.

C_P = a (P_m – P_o) L/ AE

Where C_p = the correction for pull in metres
           P = the pull applied during measurement, in newtons (N).
           P_o= the pull under which the tape is standardized in newtons (N).
            L = the measured length in metres.
            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.

P_m > P_o Correction is positive

P_m > P_o Correction is negative

4. Correction for Sag

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,

C_s = W²L / 24P²

C_s = the sag correction for a single span, in metres.

L = the distance between supports in metres.

W= weight per meter length (Total mass of the tape in kilograms) W= mgl₁

m = the mass of the tape, in kilograms per metre.

l₁ = the distance between supports in metres
P = the applied pull, in newtons (N).

5. 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 (P_m – P_o) L/ AE = W²L / 24Pm²

Pm²(P_m – P_o) ⁼ W²AE/24

The value of P is calculated by trial and error method.

6. Correction for Slope

This correction is always subtractive from the measured length.

Civil is King

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Surveying and Types of survey Work

INTRODUCTION

Primary Classification

Secondary Classification

Classification Based on Place of Survey

LAND SURVEY

HYDROGRAPHIC SURVEY

UNDERGROUND SURVEY

AERIAL SURVEY

Classification Based on PURPOSE or Objective of Survey

GEOLOGICAL SURVEY

GEOGRAPHICAL SURVEY

ENGINEERING SURVEY

CADRASTRAL SURVEY

DEFENCE SURVEY

TOPOGRAPHICAL SURVEY

ARCHEOLOGICAL SURVEY

CITY SUREVY

CONTROL SURVEYING

ASTRONOMIC SURVEYS

ROUTE SURVEY

CONSTRUCTION SURVEYS

Classification Based on INSTRUMENTS USED

CHAIN SURVEY

TRAVERSE / COMPASS SURVEY

LEVELLING SURVEY

TACHEOMETRY

PLANE TABLE SURVEY

TRIANGULATION SURVEY

EDM SURVEY

TOTAL STATION SURVEY

SATELLITE SURVEY

PRINCIPLE OF SURVEY

WORK FROM “WHOLE TO PART"

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-

CHAIN SURVEYING

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