GATE CE 2025 Set 1 — Question 64
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Geomatics Engineering → Levelling → Reciprocal Levelling
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Question
In levelling between two points A and B on the opposite banks of a river, the readings are taken by setting the instrument both at A and B, as shown in the table. If the RL of A is 150.000 m, the RL of B (in m) is _______________ (rounded off to 3 decimal places).
| Level position | Staff readings (A) | Staff readings (B) |
|---|---|---|
| A | 1.800 | 1.350 |
| B | 1.450 | 0.950 |
Correct answer
150.47 to 150.48
Solution
Reciprocal levelling is used to determine the true difference in elevation between two points when it is not possible to set up the level midway between them (e.g., across a river). This method eliminates errors due to curvature, refraction, and imperfect adjustment of the line of collimation.Let:
- be the staff readings at A and B respectively when the instrument is at A.
- be the staff readings at A and B respectively when the instrument is at B.
- m, m
- m, m
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