CHAIN SURVEYING—PURPOSE AND PRINCIPLE OF CHAIN

SURVEYING
Chain surveying is one of the methods of land surveying. Chain surveying is the simplest and at the same time most accurate method of surveying, when the area of land to be surveyed is limited. In this type of surveying, only linear measurements are taken in the field, with the help of chain. tape and offset rod.
Generally, for the work of ordinary precision the distance between the stations can be measured with chain. For the work of higher precision and to locate the details of the objects from the chain line a tape can be used. When the objects or the boundary of an area is located at a small distance. (ie, less than 2 or 3 m) from the chain line, preferably an offset rod can be used.
Chain surveying is suitably adopted, in the following situations:
1. The area of land to be surveyed is small
2. The large scale maps are to be prepared.
3. The ground is fairly level and covering with simple details.
4. The formation of well conditioned triangle is easy.
Chain surveying is unsuitable when
1. The area of land to be surveyed is large
2. The area is crowded with many details.
3. The area of land is undulated.
4. The formation of well conditioned triangles becomes difficult due to obstacles like large buildings.

Purpose of Chain Surveying

1. To fix the boundaries of the land.
2. To prepare the plan of the site.
3. To calculate the area of the plot.
4. To divide land into smaller units.
5. To collect necessary data for exact description of the land
6. To collect data required for engineering works, such as Roads, Railways, canals etc.

Principle of Chain Surveying

The principle of chain surveying is triangulation. When it is done with a chain it is called chain triangulation. According to this principle, the area to be surveyed is divided into number of small triangles connecting with each other. A triangle in which an angle is less than 30° or more than 120° is said to be ill-conditioned. III- Conditioned triangles are not preferred, because their apex points are not sharp and well defined. Hence, there is a possibility of error in plotting.

TERMS USED IN CHAIN SURVEYING

SURVEY LINES

The Lines joining the survey stations are called survey lines. The survey lines are denoted by Survey lines may be of the following types.
(i) Main Survey Lines: The lines joining the main survey stations are called ‘Main Survey Lines, The main survey lines should cover the whole area to be surveyed.
(ii) Base Line: The longest of the main survey lines of an area is known as ‘Base line’. It is a most important line in chain surveying vartous stations are plotted with reference to base line. The network of triangles are built up on it. Hence, this line should be measured more accurately.
(iii) Check Lines: : A/check line may be laid t by joining the apex of the triangle to any point on the opposite side or by joining two points on any two sides of f triangles The purpose of check line is to check the accuracy of the frame work. It is also known as proof line. Each triangle must have a check line
(iv) Tie Lines: The chain lines joining subsidiary or tie stations on the main survey lines are known as ‘tie lines’. The object of running a tie line is to locate the details, which are far away from main survey lines. It also serves the purpose of check line.

OFFSET

The lateral measurement taken from an object to the survey line is known as “offset”. Offsets are taken to locate the objects with reference to the survey line. The offsets may be of two types
(a) Perpendicular Offsets.
(b) Oblique Offsets.

1. Perpendicular Offsets: If the lateral measurements taken right angles (i.e., 90°) to the survey line, they are known a perpendicular offsets.
The following methods may be adopted to set out perpendicular offsets.
(i) A perpendicular may be set by swinging a tape from the object to the chain line. The point of minimum reading on the tape will be base of the perpendicular offset.
(ii)Setting a right angle in the ratio 3:4:5.
(iii) Setting a right angle by cross-staff or optical square

2. Oblique Offsets: Offsets not perpendicular to the chain line is said to be oblique offsets. These offsets are taken when the objects are located at a long distance from chain line.