Correct Answer - Option 4 : Back sight
Explanation:
Back sight: The first staff reading taken by the surveyor after the leveling instrument is set up and leveled. 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 the height of the instrument to determine the 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.
