GATE 2025 CE (CE1) – Question 64
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 reading on A | Staff reading on B |
|---|---|---|
| A | 1.800 | 1.350 |
| B | 1.450 | 0.950 |
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Show answer and explanation
Correct answer: 149.723 to 151.227
Explanation
This is reciprocal levelling. With the instrument at A, B is higher than A by $1.800 - 1.350 = 0.450$ m. With the instrument at B, B is higher by $1.450 - 0.950 = 0.500$ m. Reciprocal levelling cancels the errors due to curvature, refraction and collimation by taking the mean, which is 0.475 m. So the RL of B is $150.000 + 0.475 = 150.475$ m.