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.