Correct option is A
Levelling:
Levelling is the operation used (or required) to determine the relative heights of points on, above, or below the surface of the earth.
Profile Levelling:
It is a type of levelling done for the purpose of determining the elevations of the ground surface along a fixed line (such as roads, rails, rivers etc.).
Profile levelling is also called longitudinal levelling.
In this levelling, the observations (staff readings) are taken at regular intervals (at a constant horizontal distance) along the central line and foresight to a properly selected turning point.
Fly Levelling:
It is a type of differential levelling done for the purpose of determining the approximate elevation of different points.
Fly levelling is done rapidly but with low precision.
Auto Levelling:
Automatic levelling (auto levelling) is a levelling that determines the difference in elevation of points with the help of a dumpy level or a builder's auto level.
The equipment can be used for small scales such as residential building construction and large civil engineering projects.
Trignometric Levelling:
In this method of levelling in which the difference in elevations is determined indirectly from the horizontal distance and vertical angle.
As the trigonometric relations are used to determine the elevations, the method is called trigonometric levelling.
