During a levelling survey, the backsight of A was found greater than the foresight reading at point B. Which of the following is correct?

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SSC JE Civil 10 Oct 2023 Shift 1 Official Paper-I
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  1. Height of instrument is required to know which point is higher.
  2. Elevation of A is more than that of B.
  3. Elevation of B is more than that of A.
  4. Either A or B can have higher elevation.

Answer (Detailed Solution Below)

Option 3 : Elevation of B is more than that of A.
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Detailed Solution

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Concept:

Back sight (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.

Fore sight (FS):

The last staff reading taken before changing the instrument to the other position. It is the staff reading taken on 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 (IS): 

All readings taken between back sight and fore sight. These are the points whose RL is determined by the method already mentioned above in FS.

Explanation:

In levelling,

If backsight is less than foresight (BS < FS), then backsight station is at higher point than foresight station.

If backsight is more than foresight (BS > FS), then backsight station is at lower point than foresight station.

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