BS reading at BM (RL = 150 m) = 1.56 m. FS at point A taken with inverted staff = 1.65 m. Calculate RL of point A.
Correct Answer: D. 153.21 m
📚 Detailed Explanation: Inverted FS at Point A — RL of A = 153.21 m
Why D (153.21 m) is correct: The BM is a normal ground mark, so BS is positive. Point A has an inverted staff (overhead target), so FS is treated as negative. Subtracting a negative FS from HI gives RL higher than HI.
Given:
RL of BM = 150 m
BS on BM (upright staff) = 1.56 m → positive
FS at Point A (inverted staff) = 1.65 m → treated as NEGATIVE: FS = -1.65 m
RL of BM = 150 m
BS on BM (upright staff) = 1.56 m → positive
FS at Point A (inverted staff) = 1.65 m → treated as NEGATIVE: FS = -1.65 m
Calculation:
HI = RL of BM + BS = 150 + 1.56 = 151.56 m
RL of A = HI – FS = 151.56 – (-1.65) = 151.56 + 1.65 = 153.21 m
Why RL(A) > HI
| Item | Value | Reason |
|---|---|---|
| HI | 151.56 m | Line of sight is 1.56 m above BM (RL 150) |
| FS (inverted) | −1.65 m | Staff held upside down: the 1.65 m reading is ABOVE the line of sight, so FS is negative |
| RL(A) = HI − (−1.65) | 153.21 m | A is 1.65 m ABOVE the instrument line of sight; hence higher than HI |
- HI = 150 + 1.56 = 151.56 m.
- RL(A) = 151.56 − (−1.65) = 153.21 m.
- Overhead target (inverted staff) always gives RL higher than the HI.