Civil Engineering > GATE 2026 SET-1 > Levelling
The height of the plane of collimation of a levelling instrument is 100.000 m from a datum. The levelling instrument measures a Back Sight of 1.500 m on a vertically held staff at a ground point P. The reduced level (in m) of the ground point P with respect to the datum is ________________ (rounded off to three decimal places).
Correct : 98.500
In the Height of Instrument (HI) method of levelling, the reduced level of any point is found by subtracting the staff reading from the height of the plane of collimation (HI), since the staff reading represents the vertical distance below the line of sight at that point.
RL of P = HI − BS = 100.000 − 1.500 = 98.500 m
Correct answer: 98.500 ✓
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