Chain survey is the simplest method of surveying. In this survey only measurements are taken in the field, and the rest work, such as plotting calculation etc. are done in the office. This is most suitable 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 and some time cross staff.
Survey stations are of two kinds
1. Main Stations
2. Subsidiary or tie
Main stations are the end of the lines, which command the boundaries of the survey, and the lines joining the main stations re called the main survey line or the chain lines.
Subsidiary or the tie stations:
Subsidiary or the tie stations are the point selected on the main survey lines, where it is necessary to locate the interior detail such as fences, hedges, building etc.
Tie or subsidiary lines:
A tie line joints two fixed points on the main survey lines. It helps to checking the accuracy of surveying and to locate the interior details. The position of each tie line should be close to some features, such as paths, building etc.
It is main and longest line, which passes approximately through the centre of the field. All the other measurements to show the details of the work are taken with respect of this line.
A check line also termed as a proof line is a line joining the apex of a triangle to some fixed points on any two sides of a triangle. A check line is measured to check the accuracy of the framework. The length of a check line, as measured on the ground should agree with its length on the plan.
These are the lateral measurements from the base line to fix the positions of the different objects of the work with respect to base line. These are generally set at right angle offsets. It can also be drawn with the help of a tape. There are two kinds of offsets:
1) Perpendicular offsets, and
2) Oblique offsets.
The measurements are taken at right angle to the survey line called perpendicular or right angled offsets.
The measurements which are not made at right angles to the survey line are called oblique offsets or tie line offsets.
Procedure in chain survey:
The preliminary inspection of the area to be surveyed is called reconnaissance. The surveyor inspects the area to be surveyed, survey or prepares index sketch or key plan.
2. Marking Station:
Surveyor fixes up the required no stations at places from where maximum possible stations are possible.
3. Then he selects the way for passing the main line, which should be horizontal and clean as possible and should pass approximately through the centre of work.
4. Then ranging roads are fixed on the stations.
5. After fixing the stations, chaining could be started.
6. Make ranging wherever necessary.
7. Measure the change and offset.
8. Enter in the field the book.