METHODS OF LEVELLING

To determine the difference in elevation of various points following method are used:

(a) Barometric levelling       

(b) Hypsometric levelling

(c) Direct levelling and          

(d) Indirect levelling.

Barometric  Levelling

This method based on the principle that atmospheric pressure depends upon the elevation of place. To measure the atmospheric pressure barometer is used and therefore elevation is computed. However it is not precise method since the atmospheric pressure depends upon temperature and season also. It can be used in exploratory surveys.

Hypsometric Levelling

This depends on the principle that boiling point of water is decreases with the elevation of the place. Therefore the elevation difference between two points can be found by noting the difference in the 2 places in boiling point of water. This method is also useful for exploratory survey only.

Direct Levelling

In all engineering projects it is general form of levelling. In this method on a graduated staff horizontal sight is taken and the difference in the elevation of line of sight and ground at which staff is held are found. From the instrument station knowing the height of line of sight the difference in the elevations of instrument station and the ground on which staff is held may be found. This technique is thoroughly described in next article.

Indirect Methods

Instruments are used to measure the vertical angles in this method. By various methods Distance between the instrument and staffs are measured. Then by using trigonometric relations, the difference in elevation may be computed. This is not considering under the scope of this book. One can find details of such methods in books on surveying and levelling.