Correct Answer - Option 3 : back-sight
Concept:
Levelling:
- Levelling is the operation required in the determination or more strictly the comparisons of heights of points on the surface of the earth.
- While levelling –
- When instrument is being shifted the staff must not be moved.
- When staff is being carried forward, the instrument must remain stationary.
Back Sight: The first staff reading taken by the surveyor after the levelling instrument is set up and levelled. B.S is generally taken on the point of known reduced level as on the benchmark or a change point.
Fore sight (FS): The last staff reading was taken before changing the instrument to the other position. It is the staff reading taken on a point whose RL is to determined. This sight is considered as negative and deduced from Height of Instrument to determine RL of the point.
Intermediate sight: All readings taken between backsight and foresight. These are the points whose RL is determined by the method already mentioned above in FS.
