Principle of chain surveying

PRINCIPLE OF CHAIN SURVEYING

Reconnaissance Survey: Before commencing chain surveying, go round the area. Identify important locations for stations and possible obstacles. This primary investigation of the area is called Reconnaissance Survey. A sketch is prepared showing important lines to be measured. These lines should be close to the boundary. Then a Plan or Map is prepared.

1. PRINCIPLE OF CHAIN SURVEYING – Chain Triangulation

Chain Surveying consists of measuring of lengths of straight lines using Chain or Tape.

The principle of chain surveying is to divide the area of the land into a network of Triangles of suitable sides. The triangle is the simplest geometrical figure that can be plotted with the lengths of its sides alone. Hence, chain surveying is also termed as Chain Triangulation. The exact arrangement of triangles to be formed depends upon the shape of the ground and obstacles met with.

Well-Proportioned Triangles: The area to be surveyed is marked as a skeleton or framework with a number of Well-Proportioned Triangles. A triangle is said to be well-proportioned, if its angles are 30° to 120°. The error in plotting a triangle will be minimum, if it is wellproportioned. All the sides of the triangles are measured and cross-checked using Check Lines.

2. TERMINOLOGY (Fig. 12).

1. Survey Station: It is defined as the beginning point or end point of a chain line. Survey stations should be selected such that they are inter-visible. These are of two types, Main Survey Station and Tie Station or Subsidiary Station.

2. Main Survey Station: It is at the ends of main survey lines. It fixes the boundaries of the survey.

3. Tie Station or Subsidiary Station: It is the station selected on a main survey line for running auxiliary lines to locate the interior details.

4. Main Survey Lines: Lines joining the main survey stations are Main Survey Lines. These lines should run as close as possible to the boundaries to avoid long offsets.

5. Base Line: Base Line is the longest of the main survey lines. It divides the area into two halves. That is, the survey line passing through the center of the area is known as Base Line.

6. Check Line: It is the line joining any two points on the two sides of a triangle or by joining the apex of a triangle to any point on the opposite side. It is meant for checking the accuracy of the measurements made in the field.

7. Tie Line or Subsidiary Line: It is the chain line joining the tie station on the main survey line. It is used (i) to locate the interior details far away from main survey line and (ii) to check the accuracy of the frame work.

8. Offset: See Fig. 13. Lateral distance of buildings, boundaries, etc., measured from a chain line or survey line AB is called Offset.

When measurements are made perpendicular to the chain line, it is called a Perpendicular Offset. If they are not perpendicular to the chain line, then it is called an Oblique Offset.