Levelling MCQ with answer and solution

Concept

Fore sight is subtracted from height of instrument, so it is called minus sight.

Formula

RL of point = HI - FS

Step-by-step solution

1. Back sight (BS) is added to RL of benchmark to get HI. So BS is called plus sight. 2. Fore sight (FS) is subtracted from HI to get RL of the point. So FS is called minus sight. 3. Hence the answer is fore sight.

Explanation

In levelling, HI = RL + BS, so back sight is a plus sight. The RL of a new point = HI - FS, so fore sight is a minus sight. Fore sight is the last reading taken before shifting the instrument.

Why the other options are wrong

A: Inverted staff reading is taken as negative in calculation, but it is not called minus sight. B: Back sight is called plus sight. C: Intermediate sight is any reading between BS and FS. It is also subtracted, but the standard name minus sight is used for fore sight.

Shortcut

BS = plus sight, FS = minus sight.

Common mistake

Students mix up BS and FS. Remember BS is added and FS is subtracted.