Chain Surveying

• Chain surveying is the type of surveying in which only linear measurements are taken in the field with help of chains or tape and no angular measurements are taken.

• The principle of chain surveying is to divide the area into some suitable triangles.

• As a triangle is the only simple plane geometrical figure which can be plotted from the lengths of the three sides even if the angles are not known.

• Triangulation is the principle of chain surveying

1) Survey Stations

• Survey stations are the points at the beginning and the end of the chain line.

• They may also occur at any convenient position on the chain line.

• Such Stations may be a) Main Stations b) Subsidiary Stations and c) Tie Station.

a) Main Station

• Stations taken along the boundary of an area as controlling points are known as main stations.

• The line joining the main stations are called "main survey lines".

• The main stations are denoted by "△" with letters A, B, C, D etc.

b) Subsidiary stations

• Stations that are on the main survey line or any other survey lines known as 'Subsidiary Station'.

• These stations are taken to run subsidiary lines for dividing the area into triangles.

• These stations are denoted by "⊙" with letters S1, S2,S3 etc.

c) Tie Station

• These are also subsidiary stations taken on the main survey lines, lines joining the tie stations are known as 'tie lines'. The lines are taken to locate interior details.

• The stations are denoted by "⊙" with letters T1, T2, T3 etc.

2) Main Survey lines

• The lines joining the main stations are called 'main survey lines' or 'chain lines'.

3) Baseline

• The baseline on which the framework of the survey is built is known as 'baseline'. It is the most important line of the survey.

• Generally, the longest of the main survey line is considered the baseline.

• Baseline should be measured very carefully and accurately & invar tape is used to measure baseline (Reference line).

4) Check line

• The line joining the apex point of a triangle to some fixed points on its base is known as a 'check line'.

• It is taken to check the accuracy of the triangle and this line helps to locate interior details.

5) Tie line

• A line joining a tie station is called a tie line.It is run to take the interior details which are far away from the main lines and also to avoid long offsets. it can also serve as a checking line.

Well Condition Triangle

• To get good results in plotting, the framework should consist of triangles that are nearly equilateral as possible.

• A triangle is said to be well conditions if it can be plotted accurately by the interaction of arcs from the ends of the baseline.

• It must be ensured that

No angle Should be less them 30°

No angle should be greater than 120°

Best triangle has angle 56°14' each (Isoscale triangle)