Statement I): Reciprocal levelling is adopted to decide the difference of level between two points, a considerable distance apart, with great precision.

Statement II): Reciprocal levelling eliminates errors due to curvature, refraction and collimation.

Concept:

Reciprocal levelling is adopted to accurately determine the level difference between two points which are separated by obstacles like a river, ponds, lakes, etc.

It eliminates the following errors:

i) error in instrument adjustments i.e error due to collimation

ii) the combined effect of Earth's curvature and the refraction of the atmosphere

iii) variation in the average refraction

Important Points:

Reciprocal levelling eliminates the error due to collimation and error due to curvature of Earth completely, but as the refraction depends upon the atmosphere which may change every minute;