Chain Surveying

Chain survey is the simplest method of surveying. In chain survey, only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office. Here only linear measurements are made i.e. no angular measurements are made. This is most suitably adapted to small plane areas with very few details. If carefully done, it gives quite accurate results.

The necessary requirements for field work are

• Chain
• Tape 
• Ranging-Rod
• Arrows 
• Cross staff

Parts of Chains used in Surveying

The chain consists of many small parts used for handling or reading the measurements.

• At the ends chain is provided with brass handle with swivel joint so that it can be easy to roll or unroll the chain without twisting and knots.
• At every 10th link is provided with a tally of one teeth, 20th link with a tally of two teeth and so on till 40th link. This is provided for the easy reading of measurements.
• At the center of the chain is provided with a circular talley used for easy reading.

 

Types of Chains used in Surveying

Depending upon the length of the chain, these are divide into following types,

1. Metric chains
2. Steel band or Band chain
3. Gunter’s chain or surveyor’s chain
4. Engineer’s chain
5. Revenue chain

A. Metric chains

Metric chains are the most commonly used chain in India. These types of chains comes in many lengths such as 5, 10, 20 and 30 meters. Most commonly used is 20m chain. Tallies are provided at every 2m of the chain for quick reading. Every link of this type of chain is 0.2m. The total length of the chain is marked on the brass handle at the ends.

B. Steel band or Band chain

These types of chain consist of a long narrow strip of steel of uniform width of 12 to 16 mm and thickness of 0.3 to 0.6 mm. this chain is divides by brass studs at every 20cm or instead of brass studs, band chain may have graduated engraving as centimeter.

For easy use and workability band chains are wound on steel crosses or metal reels from which they can be easily unrolled. These steel bands are available in 20m and 30m length and the width of about 12-16mm.

C. Gunter’s chain or surveyor’s chain

Gunter chain comes in standard 66ft. These chain consists of 100links, each link being 0.66ft or 7.92inches. The length 66ft is selected because it is convenient in land measurements.

10 square Gunter’s chains = 1 Acre

10 Gunter chains = 1 Furlong

80 Gunter chains = 1 mile

D. Engineer’s chain

This chain comes in 100ft length. Its consist 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.

E. Revenue Chain

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

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,

5m chain = + or – 3mm

10m chain = + or – 3mm

20m chain = + or – 5mm

30m chain = + or – 8mm

Chain length shorten due to

1. Bending of links.
2. Sticking of mud in the rings

Chain length increases due to

• Opening of small rings.
• Wearing of surfaces.

Chains may be tested with respect to

• Steel tape
• Permanent test gauge
• Pegs driven in the field at required distances
• Permanent test gauge made with dressed stones

If chain is found long, then

• Close the joins of the rings
• Reshape the elongated rings
• Remove one or two rings
• Replace worn out rings

If chain is found short, then

• Straighten the links
• Replace the small rings with big one
• Insert additional rings
• Flattening the circular rings

Errors in chain Surveying

Errors in chaining may be classified as:

• Personal errors
• Compensating errors, and
• Cumulating errors.

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.

Compensating Errors

These errors may be sometimes positive and sometimes negative. Hence

They are likely to get compensated when large number of readings are taken. The magnitude of such errors can be estimated by theory of probability. The following are the examples of such errors:

• Incorrect marking of the end of a chain.
• Fractional part of chain may not be correct though total length is corrected.
• Graduations in tape may not be exactly same throughout.
• In the method of stepping while measuring sloping ground, plumbing may be crude.

Cumulative Errors

The errors that occur always in the same direction are called cumulative errors. 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:

1. Bad ranging
2. Bad straightening
3. Erroneous length of chain
4. Temperature variation
5. Variation in applied pull
6. Non-horizontality
7. Sag in the chain, if suspended for measuring horizontal distance on a sloping ground.

Errors (i), (ii), (vi) and (vii) are always +ve since they make measured length more than actual.

Advantages and Disadvantages of Chains in Surveying

Advantages of Chains in Surveying

• Chain survey is simplest and commonest method used in surveying exercises
• The equipment used to conduct chain survey are simple to use,
• The equipment used in chain survey can easily be replaced. For example measuring rods can be replaced with measuring tape.
• This method does not involve complicated mathematical calculation. I know this is the relief to those who are afraid of mathematics
• In chain survey few people are needed to conduct the survey. Normally chain survey team has three people Booker, leader and follower.

Disadvantages of Chains in Surveying

• Simple chain survey cannot be conducted in built up areas and large areas.
• Simple chain survey is subject to several chances of errors of accumulation which may cause by problem of chain. The chain linkage may fail to stretch up properly and result in inaccurate data. Also clogging of chain may read to error in reading.
• It is time consuming
• It may not be conducted in areas with steep slopes or water logged areas. Chain survey is usually conducted in dry areas with gentle slopes. It becomes more complicated when survey is conducted in areas that are too wet.
• Chain survey becomes more complicated method when there are raised points (obstacles) in between areas to be surveyed

Principles of chain Surveying.

(i) First of all, the site should be inspected with a view to find a suitable location for stations.

(ii) The survey lines should be as few as practicable and such that the framework may be plotted.

(iii) If possible, a base line should be run roughly through the middle of the area on which the framework of triangles covering the major portion of the area may be built up.

(iv) All the triangles should be well conditioned, i.e., no angle should be less than 30° or greater than 120° in a triangle.

 

(v) Each portion of the survey should be provided with check lines.

(vi) The offsets should be short; particularly for locating features which are important.

A number of subsidiary lines or tie lines should be run to locate the details and to avoid long offsets.

(vii) As few lines as possible should be run without offsets.

(viii) The obstacles to ranging and chaining should be avoided as far as possible.

(ix) The lines should lie as far as possible on the ground level.

(x) In lines lying along a road, the possibility of interruption during chain surveying, a line at one side of the road should be drawn.

(xi) The main Stations should be inter-visible and the main principle of surveying, i.e., working from the whole to the part, should be strictly observed.

(xii) The lines should be measured in an order avoiding unnecessary walking between stations.

Chain Surveying Procedure:

For chain surveying, at least two men are required, but frequently three people are employed.

They are:

(1) The surveyor, who does the reading and booking,

(2) The leader, and

(3) The follower.

(i) To start the chaining of a line the follower holds the zero end of the chain in contact with the peg at the beginning of the line and presses the handle with his feet and stands firmly over it.

(ii) The leader holds another end of the chain and goes along with the arrows and ranging rods on the line.

(iii) Nearly at the end of the chain length, he stops and aligns with the help of ranging rod which he keeps vertical and faces the follower, who gives him instructions by his arms.

(iv) After alignment, the leader pulls the chain and inserts an arrow in to the ground to mark the end.

(v) The lateral measurements or offsets are taken from the chain line to any object that is to be plotted on the plan.

(vi) The chain line should be such that these offsets are as short as possible. While pulling the chain, care should be taken.

(vii) After taking the offsets, the leader picks up the staff rod and remaining arrows keeping the chain a little away from the line so that the arrow placed is not disturbed, starts moving ahead as before.

Watch the Video Below for Better understanding.

(viii) As the follower reaches the arrow with the near end of the chain, he should speak loud “chain” or “tape” to give a warning to the leader that he has nearly reached the arrow or a chain length and immediately the leader stops.

(ix) The follower holds the handle against the arrow and directs the leader to come in line as before.

(x) The leader again stretches the chain and fixes the arrow in the ground at another chain length or make a cross if the ground is firm.

(xi) Again, the leader walks in the line in the same manner and the follower now picks up the first arrow, comes to the second arrow and gives instructions for the third chain length.

(xii) Thus, the whole process is repeated until the end of the line is reached.

(xiii) The number of arrows with the follower is an indication of the number of full chain lengths completed at any time.

(xiv) After some time the number of arrows should be checked mutually by the follower and the leader so that no chain length is missed and no arrow is lost.

Generally, the number of arrows taken is ten and hence after fixing the tenth arrow, the leader speaks out “arrows” which means that this was the tenth chain line.

(xv) The follower then goes to the tenth arrow and picks it up after fixing a ranging rod there. The arrows are then handed over to the leader, and a record is made in the field book by the surveyor.

(xvi) For the fractional length of the Chain, the leader stretches the chain beyond the end station. While the follower holds the rear handle of the chain against the last arrow.

The leader reads the fractional chain length loudly, and the surveyor notes the entire length of the line.

Duties of the follower and leader During Chain Surveying.

The duties of the follower  (Chain man at the rear end of the chain) are;

(i) To give signals and instructions to the leader.

(ii) To place the leader in the line of the ranging rod.

(iii) To carry the rear handle.

(iv) To pick up the arrows.

The duties of the leader ( the chain man at the forward end or head ) are;

(i) To stretch the chain forward.

(ii) To insert the arrow at every chain length.

(iii) To obey the directions given by the follower.

Recording the measurements in the Field Book.

The field book is an oblong book with a hinge at the narrow edge, and the chain is represented in it by one or two red lines or blue lines ruled down centrally along the length of each page.

The booking or recording of the field work is commenced from the bottom of the first page. The double line book is better because the main chain line readings are separated from offset readings.

The station points are lettered or numbered, and a small rectangle or triangle is drawn in the field book to enclose the chaining figure at the station points.

 

The lines meeting at the station point are also marked, and the reference sketches are drawn on field book, and after this line survey, lines are run by chaining.

When a chain survey is to be conducted the necessary equipment should be taken, and reconnaissance or preliminary inspection, of the area, should be made.

By this inspection, the surveyor can judge the network.

Wooden pegs & ranging rods mark the station points.

Then, the stations are marked the reference sketches are drawn on field book, and after this, the survey lines are run by chaining.

Precautions.

The following points should be kept in view while booking the field notes.

(i) All the measurements should be recorded as soon as they are taken.

(ii) Each chain line should be recorded on a separate page of the field book.

(iii) Figuring and writing should be neat, and legible overwriting of the figures should be avoided completely.

(iv) The notes should be complete, and nothing should be left to memory.

(v) Notes should be so full and neat that the draftsman who is unfamiliar with the area surveyed may plot easily

(vi) Neat reference sketches should be given in the field book, and explanatory notes should be added.

(vii) The field book should be kept clean, and no entry should be made in it, nor it should be rubbed.

If an entry is wrong, a line should be drawn through it, and the count entry is made over it.

if an entire page of the Field book is to be discarded, it should be crossed and marked canceled and reference of the other page in which the correct entries are made should be given on the canceled page.

Levelling

Levelling

Levelling (or Leveling) is a branch of surveying, the object of which is: i) to find the elevations of given points with respect to a given or assumed datum, and ii) to establish points at a given or assumed datum. The first operation is required to enable the works to be designed while the second operation is required in the setting out of all kinds of engineering works. Levelling deals with measurements in a vertical plane.

Level surface: A level surface is defined as a curved surface which at each point is perpendicular to the direction of gravity at the point. The surface of a still water is a truly level surface. Any surface parallel to the mean spheroidal surface of the earth is, therefore, a level surface.

Level line: A level line is a line lying in a level surface. It is, therefore, normal to the plumb line at all points.

Horizontal plane: Horizontal plane through a point is a plane tangential to the level surface at that point. It is, therefore, perpendicular to the plumb line through the point.

Horizontal line: It is a straight line tangential to the level line at a point. It is also perpendicular to the plumb line.

Vertical line: It is a line normal to the level line at a point. It is commonly considered to be the line defined by a plumb line.

Datum: Datum is any surface to which elevation are referred. The mean sea level affords a convenient datum world over, and elevations are commonly given as so much above or below sea level. It is often more convenient, however, to assume some other datum, specially, if only the relative elevation of points are required.

Elevation: The elevation of a point on or near the surface of the earth is its vertical distance above or below an arbitrarily assumed level surface or datum. The difference in elevation between two points is the vertical distance between the two level surface in which the two points lie.

Vertical angle: Vertical angle is an angle between two intersecting lines in a vertical plane. Generally, one of these lines is horizontal.

Mean sea level: It is the average height of the sea for all stages of the tides. At any particular place it is derived by averaging the hourly tide heights over a long period of 19 years.

Bench Mark: It is a relatively permanent point of reference whose elevation with respect to some assumed datum is known. It is used either as a starting point for levelling or as a point upon which to close as a check.

Methods of levelling

Three principle methods are used for determining differences in elevation, namely, barometric levelling, trigonometric levelling and spirit levelling.

Barometric levelling

Barometric levelling makes use of the phenomenon that difference in elevation between two points is proportional to the difference in atmospheric pressures at these points. A barometer, therefore, may be used and the readings observed at different points would yield a measure of the relative elevation of those points.

At a given point, the atmospheric pressure doesn’t remain constant in the course of the day, even in the course of an hour. The method is, therefore, relatively inaccurate and is little used in surveying work except on reconnaissance or exploratory survey.

Trigonometric Levelling (Indirect Levelling)

Trigonometric or Indirect levelling is the process of levelling in which the elevations of points are computed from the vertical angles and horizontal distances measured in the field, just as the length of any side in any triangle can be computed from proper trigonometric relations. In a modified form called stadia levelling, commonly used in mapping, both the difference in elevation and the horizontal distance between the points are directly computed from the measured vertical angles and staff readings.

Spirit Levelling (Direct Levelling)

It is that branch of levelling in which the vertical distances with respect to a horizontal line (perpendicular to the direction of gravity) may be used to determine the relative difference in elevation between two adjacent points. A horizontal plane of sight tangent to level surface at any point is readily established by means of a spirit level or a level vial. In spirit levelling, a spirit level and a sighting device (telescope) are combined and vertical distances are measured by observing on graduated rods placed on the points. The method is also known as direct levelling. It is the most precise method of determining elevations and the one most commonly used by engineers.

Levelling Instruments

The instruments commonly used in direct levelling are:

1. A level
2. A levelling staff

Theodolite

Theodolite

Theodolite has many parts which needs to be adjusted every time while surveying. It is important to know about theodolite parts and their functions before using it to minimize errors during theodolite surveying.

Theodolite is an instrument used in surveying to measure horizontal and vertical angles. It is also used for leveling, indirect measure of distances and prolonging a line etc. The line of sight of theodolite can be rotated through 180o in vertical plane about its horizontal axis.

Parts of Theodolite and their Functions

Following are the parts of a theodolite:

• Telescope
• Vertical circle
• Index frame
• The standards
• The upper plate
• The lower plate
• The leveling head
• The shifting head
• Plate level
• Tripod
• Plumb bob
• Magnetic compass

Telescope

A telescope is a focusing instrument which has object piece at one end and eye piece at the other end. It rotates about horizontal axis in vertical plane. The graduations are up to an accuracy of 20’.

Vertical Circle

Vertical circle is fitted to telescope and moves simultaneously with telescope. It has graduation in each quadrant numbered from 0 to 90degrees.

Index Frame

It is also called as t-frame or vernier frame. It consists two arms vertical and horizontal. Vertical arm helps to lock the telescope at desired level and horizontal arm is useful to take the measurements of vertical angles.

 

The Standards

The standards are the frames which supports telescope and allow it to rotate about vertical axis. Generally, these are in letter A-shape. So, standards are also called as A-frame.

The Upper Plate

This is also called as vernier plate. The top surface of upper plate gives support to the standards. It also consists an upper clamping screw with respect to tangents screw which helps to fixing it to the lower plate.

When the upper clamping screw is tightened both upper and lower plates are attached and moved together with some relative motion because of upper tangent screw. The upper [late also consists two verniers with magnifiers which are arranged diagonally. It is attached tow inner spindle.

The Lower Plate

This is also called as scale plate. Because it contains a scale on which 0 to 360 readings are graduated. It is attached to the outer spindle and consists lower clamping screw. If lower clamp screw is loosened and upper clamp screw is tightened, both plates can rotate together.

Similarly, if lower clamping screw is tightened and upper clamp is loosened then, only upper plate is movable and lower plate is fixed with tribratch plate.

The Leveling Head

The leveling head contains two parallel triangular plates called as tribratch plates. The upper one is known as upper tribratch plate and is used to level the upper plate and telescope with the help of leveling screws provided at its three ends. The lower one is called as lower tribratch plate and is attached to the tripod stand.

The Shifting Head

Shifting head also contains two parallel plates which are moved one over the other with in small area. Shifting head lies below the lower plate. It is useful to centering the whole instrument over the station.

Plate Level

Plate levels are carried by the upper plate which are right angles to each other with one of them is parallel to trunnion axis. These plate levels help the telescope to settle in exact vertical position.

Tripod

Tripod is nothing but a stand on which theodolite is mounted. It should place in such a way that theodolite should be in exact leveled position. The tripod has legs with steel shoes at their ends. These hold the ground strongly without any movement when placed.

Tripod has an external screw which helps to attach the theodolite by tribratch plate in fixed position.

Plumb Bob

Plumb bob is tool having a cone shaped weight attached to a long thread. The weight is hanged using thread from the center of tripod stand and centering of theodolite is done.

Magnetic Compass

Simpler theodolites may contain circular compass box in the center of upper plate. When we select north as reference meridian it will be useful

Working of Vernier Theodolite

Working of a Vernier theodolite starts with setting up and adjustment of theodolite for the measurement of angles.

Temporary Adjustment of Theodolite

Following are the adjustment required for theodolite before using it:

• Setting
• Centering
• Leveling
• Focusing

Setting of Theodolite

Setting means attaching theodolite to the tripod stand. The tripod is unfolded and placed on the ground with all legs in same level. In this operation, we level the tripod by just eye estimation. From the bottom of theodolite, a plumb bob is suspended to set the instrument over station mark.

Centering

Centering is the process in which the vertical axis of the instrument is coincided with the plumb line. The legs of tripod stand should be of equal height on leveled ground and they have cone shape metal shoes which can easily hold the ground without any movement.

 

Leveling of Theodolite

Leveling of theodolite is completed by making the vertical axis of instrument truly vertical. It is done by centering the bubble in the level tube by rotating foot screws provided. The bubbling should be done in all direction of telescope and the centered bubble should not move while taking reading.

Focusing

Focusing is the adjustment of telescope to obtain clear visibility of image through eyepiece and objective lens. This process done by removing parallax error by proper focusing of eyepiece and objective lens.

Measurement of Horizontal Angle using Theodolite

After the temporary adjustment of theodolite, the telescope is located at a point from which we must focus the other point to find horizontal angle between them. Let us say A, B and C are three points on same ground level with some distances as shown in figure below. Now the angle ABC can be determined as below described procedure.

Procedure for Measurement of Horizontal Angle

1. The instrument is located at point B with all temporary adjustments and ranging rods are located at points A and C.
2. Now, set the Vernier A reading to zero and Vernier B reading to 180 by rotating telescope. This should be done by releasing the upper clamp screw and tightening of lower clamp screw.
3. After setting the Vernier, tighten the upper clamp screw and check the reading without any error with the help of upper tangent screw.
4. Whenever the upper clamp is tightly fixed, loosen the lower clamp screw it will allow the telescope to turn without any change in reading. Now focus the ranging rod at location A and bisect it.
5. After bisecting A, release the upper clamp screw and rotate the telescope in clockwise direction towards ranging rod at C and bisect it. Now observe the Vernier reading which will be the horizontal angle between A and C or angle ABC.
6. In the above explained case, the vertical circle is present left side of the telescope, then repeat the same procedure with vertical circle right side of telescope. The average value of angles obtained in face left and face right conditions gives more accurate value of horizontal angle.

Measurement of Vertical Angle using Theodolite

1. After the temporary adjustment of theodolite, set Vernier reading of vertical circle to 0 using clamp screws and focus it to C.
2. Center the bubble using foot screws without any collimation error. Then raise the telescope slowly towards point A as shown in fig. bisect the point A and note down the Vernier reading angle AOC.
3. Similarly bisect the point B from C after which will give angle BOC. Now we can compute the angle AOB.
4. Conduct the procedure in both face left and face right conditions and the average value is considered as accurate vertical angle