Principles of surveying

PRINCIPLES OF SURVEYING

The basic principles by which various surveying methods carried out are as follows:

1. Working from 'The Whole to the Part

To survey a vast area such as a town, first a system of Control Points is established to form  the boundary lines of the area. The control points may be established by triangulation or by running a traverse surrounding the area. Then, the main triangles and traverses are broken into smaller ones and measured.

2. Location of a New Point by at least Two Independent Processes

The use of two independent processes to locate a new point helps in taking one set of measurements (linear or angular) from one process. The same is checked by another set of measurements. Using this principle, first two reference points (called Control Points) A and B are chosen on the ground. The distance AB is measured. The required point C is located by making measurements using any one of the following Direct Methods:

(i). By measuring the distances AC and BC: See Fig. 2. In this method, the linear measurements AC and BC alone are measured. Distance AB is known. Locate C by drawing arcs from A and B with the radii AC and BC respectively. This method is used in Chain Surveying.

(ii) By measuring the distance BC and the angle ABC: See Fig. 3. In this method, both linear and angular measurements are made. The distance BC and the included angle ABC (0) are measured. AB is known. Locate C by drawing an arc of length BC on the line drawn from B with the angle 0. This method is used in Traverse Survey.

(iii) By measuring the included angles ABC and BAC: See Fig. 4.

In this method, angular measurements alone are made. The included angles ABC (01) and BAC (02) are measured. Locate C by the intersection of the two angular lines drawn from B and A with angles Oand O2 respectively. This method is used in Triangulation.

(iv) By drawing a perpendicular from C on AB: See Fig. 5.

In this method, a linear measurement alone is made, though it involves the angular measurement (i.e., 90°) for perpendicularity. This perpendicular from C on AB is drawn using cross-staff or swinging the tape along AB. After fixing D on AB, measure AD or BD and CD. Locate C by drawing an arc from D on the perpendicular drawn from D with the length CD. This method is used in Chain Surveying.