Correct Answer - Option 3 : Reciprocal levelling
Explanation:
Reciprocal levelling:
(i) Reciprocal levelling is used to determine the correct difference of elevations of the two points which are quite apart and when it is not possible to set up the instrument mid-way between the two points to balance the fore sight and back sight.
(ii) this reciprocal levelling is usually used to determine the difference of elevations of two points on the opposite banks of a river or a deep gorge.
(iii) Reciprocal levelling eliminates the error due to curvature and refraction.
Important points:
Differential levelling: It is used when points are situated quite apart, also called as compound levelling.
Fly levelling: It is done where rapidly but low precision is required. Generally used in reconnaissance.
Trigonometric levelling: In which vertical elevations are determined by measuring horizontal distances and elevations angles.
