Correct option is B
Backsight (BS):- 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.
Foresight (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 site is considered as negative and deduced from Height of Instrument to determine RL of the point.
Intermediate sight (IS):- All readings were taken between backsight and foresight. These are the points whose RL is determined by the method already mentioned above in FS,
Change Point (CP) or Turning Point (TP):- This is a point on which both fore sight and back sights are taken.
• Staff held at this point first fore sight is taken, then instrument is shifted to new point and back sight is taken on the same point.
• Since there is a time gap between taking of fore sight and back sight, it is advisable to take well-defined point as change point
