Question

In a levelling survey work, the first staff reading taken to a staff station after shifting the instrument and the last staff reading taken to a staff station before the instrument is shifted are, respectively,
1. back sight and intermediate sight
2. intermediate sight and back sight
3. foresight and back sight
4. back sight and fore sight

Answer 1

Correct Answer - Option 4 : back sight and fore sight

Explanation:

• Levelling operation includes, the line commences with a backsight and closes with a foresight.
• The first staff reading taken after setting up of instrument is always a Backsight and last staff reading taken before changing the instrument to the other position is always a Foresight.

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.

In levelling, 

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

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