What is Chain Surveying? Principles, Well-Conditioned and Ill-Conditioned Triangles

Chain surveying is the branch of surveying in which horizontal distances are measured using a chain or tape. The operation is called chaining or taping. Mostly, a chain is used to measure the length of the line, called the chain line, and tape is employed to measure perpendicular distances to the chain line, called as offset line. 

During the process of chaining, the surveying team consists of a leader, a follower, and an assistant.  The leader is the surveyor at the forward end of the chain, and the follower is the surveyor at the rear end of the chain. The assistant helps to establish immediate points during the survey.    

The main objectives of chain surveying are:

• To determine linear measurements
• To make a plan of the area to be surveyed

Chain Surveying Applications

• To measure a moderately small, level area with the fewest obstacles
• To fill in the details between already established control points

Basic Terms in Chain Surveying

Chain surveying can be defined as the method of surveying the land area by dividing the area into triangles. Hence, the principle of chain surveying is Triangulation. The triangles developed must be WELL-CONDITIONED TRIANGLES. 

In chain surveying, a CHAIN or a TAPE is used to measure the sides of the triangles. Along with this ANGULAR MEASUREMENTS are also taken. The accuracy of chain surveying is controlled by the LINES & CHECK LINES.

Point to Remember: If the three sides of the triangle are known, there is no need to determine the angle of inclination of the triangles.

 

Principle of Chain Surveying

The principles of chain surveying are:

1. The whole area that needs to be surveyed is divided into a skeleton of a framework that consists of a number of well-connected networks of well-conditioned triangles. This is the principle of Triangulation.
2. The details are located with respect to the sides of a triangle or any subsidiary lines that is running between the sides by taking lateral measurements called offsets.

             

Chain Surveying- Suitability & Unsuitability

SUITABILITY	UNSUITABILITY
More or less leveled ground surface	Crowded Area
Area of surveying is small	Area with too many undulations
Preparation of small-scale map	Area is very large
Well-conditioned the triangle can be formed easily	Difficulty in the formation of well-conditioned triangles

Large-Scale and Small-Scale Maps

A map is said to be a large scale when 1cm of the map represents a small distance. Say,

1cm = 1m, i.e. RF = 1/100;

A map is said to be a small scale when 1cm of the map represents a small-scale map.

1cm = 100m; i.e. RF = 1/10000;

Points to Remember: A map with RF value less than 1/500 is called as LARGE-SCALE MAPS & a map with RF value greater than 1/500 is called as SMALL-SCALE MAPS.

Well-Conditioned Triangles and Ill-Conditioned Triangles

The triangles with angle(A) in the range 30<A<120 is called as WELL-CONDITIONED TRIANGLES. An equilateral triangle is the best well-conditioned triangle (Ideal Triangle) possible.

The well-conditioned triangle has perfect apex points that are sharp and are located by single ‘dot’. This will bring no possibility of relative displacement of the plotted point.

An ill-conditioned triangle is a triangle with an internal angle (A) in the range 120<A<30.

Points to Remember: Chain Surveying does not employ ill-conditioned triangles. These triangles have no perfect apex points which are neither sharp nor well-defined. Any slight displacement results in large errors in plotting.

Read More:

1. Reconnaissance Survey and Index Sketch in Chain Surveying
2. Linear Measurements in Surveying