Chain surveying is the easiest type of survey in which area to be surveyed is divided into a number of triangles.
Chain surveying instruments:
- Ranging Rods
- Offset Staff
- Optical Square
Different types of Chain
- Metric Chain
- Revenue chain
- Steel Band or Band Chain
- Engineer’s Chain
- Gunter’s Chain
Procedure of Chain surveying
- Field work in Chain surveying: In includes reconnaissance. Selection of the station, measurement of lines and taking offsets of different objects in the field
- Reconnaissance: This is the preliminary survey in which to the survey party will examine the plot to be surveyed in order to know as to how the works can be executed in the best possible ways. The party will note all details like roads, buildings, canals, ditches, culverts and the difficulties and obstacles that may arise during the carrying out of the work. The party should locate the suitable points for stations by driving pegs. Sometimes a small triangle or a circle is made around the stations and the pegs are inserted into the centres. The party should then make a rough sketch of the plot showing the possible stations and from there the arrangement of different lines. It is important to give a north line on the rough sketch and though the sketch is not prepared according to the scale, it should represent the approximate positions of the different things in the plot and hence to be a good guide for further work.
- Stations: These are points on the ground fixed by driving pegs. Every station should be located with respect to three permanent objects i.e. the distances from these objects to the stations should be measured very accurately and recorded in the field book. The advantage of taking this measurement is that if in future the peg at the station is lost, then it can be located again by knowing descriptions and distances of these objects. The selection of a particular station depends upon the fallowing important considerations:
- The triangle should be a well-defined one, i.e. nearly equilateral triangle.
- Every main station should be visible from the other two.
- There should be a minimum number of obstacles in Ranging and chaining.
- The chain line should run near the boundary of the plot.
- The chain line should be as few as possible.
- The chain line should be over an approximately levelled ground In the case of chaining along the road, it is always better to run chains on one side of the road to avoid interruptions by vehicles.
- It is better not to cross the road frequently.
- Offsets should not exceed one chain.
- Check and tie lines should be provided in sufficient number so that all the main lines, of sets and other details, can be checked thoroughly.
- Tie lines and check lines: A tie line is one which connects two points on the two main lines of the triangle. It helps in taking offsets of the objects falling within the triangle and which are too far away from the main line. A checking line is also a tie line which helps in checking the accuracy of the work after plotting in a drawing sheet. A checking line or tie line is never extended beyond the main lines.
- Measurement of lines and taking offsets: In Fig the main station A is located with respect to three permanent objects and a ranging rod is fixed to the station. One ranging rod is fixed at main station B and another at an intermediate point in between A & B. The three rods will be in a straight line when only the intermediate rod is visible if a man looks from A to B. Now measurement of line AB is taken by the chain. The chain should be properly stretched so that there is no sag in it. As the measurement proceeds, offsets are taken on both side’s of the line AB and recorded in the field book. In this way, all the lines including tie and check lines are measured and offset taken and recorded in the field book.
Advantages and disadvantages of chain Survey
- This type of survey work is suited for a small plain ground.
- It requires simple instruments.
- Plotting of maps is very simple and easy.
- But this type of surveying is not suitable for undulation land where chaining operation is tedious and subject to errors. This method is not generally recommended for a crowded city with a large number of buildings and obstacles because it cannot be divided into well-conditioned triangles. In the case of route surveying.
- e the survey work of a road, irrigation canal, railways, water and sewer lines, tunnelling etc., this method is not recommended at all.