Methods of Levelling in Surveying
There are several methods of levelling in surveying to determine the elevation of various points. Let's look at them one by one :
DirectLevelling in Surveying
This is the most common type of levelling in surveying. In this method, a spirit level fixed to the telescope of a levelling instrument is used to make the line of sight horizontal. Then all the vertical distances are measured with respect to this horizontal line of sight. These vertical distances are used to determine the difference in elevations of various points. Spirit levelling is another name for direct levelling.
Trigonometric Levelling in Surveying
Trigonometric levelling is one of the indirect methods of levelling. This is the method of levelling in which the difference in elevations is determined indirectly from the horizontal distance and the vertical angle. As the trigonometric relations are used to determine the elevations, the method is called trigonometric levelling. Trigonometric levelling is employed when the direct method on the ground becomes cumbersome. For example, when we have to find the elevation of a mountain peak, top of towers, etc.
Fig.1. Trigonometric levelling
So, knowing the horizontal distance from the level station and the angle of elevation, the height “h” can be calculated using simple trigonometry.
Suppose the angle of elevation is “𝞡” and the horizontal distance is “D”, then the height “h” can be given as :
h = D tan𝞡
BarometricLevelling in Surveying
Barometric levelling is also a type of indirect levelling technique. In this method, elevations are determined from the changes in atmospheric pressure. Atmospheric pressure decreases with increase in elevation. Generally, an aneroid barometer is employed for the purpose of calculating changes in atmospheric pressure.
Hypsometric Levelling in Surveying
Hypsometric levelling is also a type of indirect method. The elevation in this method is calculated from the changes in boiling point. The boiling point tends to decrease as the altitude increases.