Principles of Chain Survey
A triangle is a simple figure which can be plotted from the lengths of three sides even if the angles are not known. In chain survey, the area to be surveyed is divided into a framework consisting of triangles.
Selection of survey station and Survey lines
I. The main station should be mutually invisible so that ranging can be done easily.
II. Survey lines should be as few as possible so that framework of triangles can be plotted easily.
III. Survey lines should pass through a level open ground as far as possible so that chaining can be done easily.
IV. The frame should have at least one long baseline that runs through the middle of the area.
V. The main survey lines should form well conditioned triangles with no angles less than 30 degrees or more than 120 degrees.
VI. The survey lines should be selected such that there are no obstacles in chaining and ranging.
VII. The survey lines will be close to the details to avoid long offsets.
VIII. If required, number of subsidiary stations should be selected and subsidiary lines run to avoid long offsets.
IX. Each triangle should have a check line to detect mistakes in measurements and plotting.
X. While selecting main stations and survey lines, the basic principles of surveying of working from whole to the part should be followed.
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:
• 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.