Staff reading at B = 1.95 m and staff reading at C = 1.55 m from the same instrument position. The rise from B to C is:
Correct Answer: C. 0.4 m
📚 Detailed Explanation: Staff Reading at C = 0.4 m Above B (Rise = 0.4 m)
Why C (0.4 m) is correct: When the staff reading decreases from one point to the next, it means the ground has risen (the point is higher). The difference 1.95 − 1.55 = 0.40 m represents a rise of 0.4 m from B to C.
Given:
Staff reading at B = 1.95 m (intermediate or foresight)
Staff reading at C = 1.55 m (next reading)
Staff reading at B = 1.95 m (intermediate or foresight)
Staff reading at C = 1.55 m (next reading)
Rise from B to C:
Rise = Staff at B – Staff at C = 1.95 – 1.55 = 0.40 m
If RL of B = some value, then:
RL(C) = RL(B) + Rise = RL(B) + 0.40 m
Interpretation:
Smaller staff reading → ground is HIGHER (less distance from line of sight to ground)
1.95 → 1.55: reading decreased by 0.40 m → ground ROSE by 0.40 m
Rise and Fall Logic
| Comparison | Result | Ground Movement |
|---|---|---|
| Next reading < previous reading | Difference = Rise | Ground goes UP (towards the line of sight) |
| Next reading > previous reading | Difference = Fall | Ground goes DOWN (away from line of sight) |
| 1.55 < 1.95 | Rise = 1.95 − 1.55 = 0.40 m | C is 0.40 m HIGHER than B |
- Rise = larger staff − smaller staff (when reading decreases, ground rises).
- Rise from B to C = 1.95 − 1.55 = 0.40 m.
- RL(C) = RL(B) + 0.40.