Staff reading at point A = 2.8 m and at the benchmark = 2.5 m. RL of benchmark = 100 m. Calculate RL of point A.
Correct Answer: B. 99.7 m
📚 Detailed Explanation: RL of Point A = 99.7 m
Why B (99.7 m) is correct: The benchmark has RL = 100 m with a BS of 2.5 m on it. This establishes the HI. The staff at A reads 2.8 m (FS or IS), and RL(A) = HI − 2.8 = 99.7 m.
Given:
RL of Benchmark = 100 m
Staff reading at BM = 2.5 m → Back Sight (BS)
Staff reading at Point A = 2.8 m → (fore sight or IS)
RL of Benchmark = 100 m
Staff reading at BM = 2.5 m → Back Sight (BS)
Staff reading at Point A = 2.8 m → (fore sight or IS)
Calculation:
HI = RL of BM + BS = 100 + 2.5 = 102.5 m
RL of Point A = HI – Staff at A = 102.5 – 2.8 = 99.7 m
Why the Reading at A Is Larger Than at BM
| Reading | Value | Interpretation |
|---|---|---|
| BS at BM | 2.5 m | BM is below the line of sight by 2.5 m |
| FS/IS at A | 2.8 m | A is below the line of sight by 2.8 m → A is lower than BM |
| Rise/Fall | Fall = 2.8 − 2.5 = 0.3 m | A is 0.3 m lower than BM → RL(A) = 100 − 0.3 = 99.7 m ✓ |
- HI = 100 + 2.5 = 102.5 m.
- RL(A) = 102.5 − 2.8 = 99.7 m.
- Fall = 2.8 − 2.5 = 0.3 m; RL(A) = 100 − 0.3 = 99.7 m (cross-check ✓).