RL of BM at Point A (lintel bottom) = 101.50 m. Inverted staff on BM = 2.25 m; staff at Point B (ground) = 1.05 m. After instrument shift, BS at B = 1.35 m; inverted staff at Point C (canopy bottom) = 4.100 m. Find RL of Point C.
📚 Detailed Explanation: Inverted Staff Calculation — RL of Point C = 103.65 m
Step 1: Setup 1 (instrument somewhere between A and B)
RL of BM at A (lintel bottom) = 101.50 m
Inverted staff on A → BS1 = -2.25 m
HI1 = RL(A) + BS1 = 101.50 + (-2.25) = 99.25 m
Step 2: Normal staff reading at B (ground level)
FS1 = +1.05 m (upright staff on ground)
RL(B) = HI1 – FS1 = 99.25 – 1.05 = 98.20 m
Step 3: Instrument shifted; B becomes change point
BS2 = +1.35 m (upright staff on B, now acting as BM)
HI2 = RL(B) + BS2 = 98.20 + 1.35 = 99.55 m
Step 4: Inverted staff at C (bottom of canopy — overhead)
FS2 = -4.100 m (inverted)
RL(C) = HI2 – FS2 = 99.55 – (-4.100) = 99.55 + 4.100 = 103.65 m
Summary Table
| Station | BS | FS | HI | RL | Remark |
|---|---|---|---|---|---|
| A (lintel) | −2.25 | 99.25 | 101.50 | BM; inverted BS | |
| B (ground) | +1.35 | +1.05 | 99.55 | 98.20 | Change Point |
| C (canopy) | −4.100 | 103.65 | Inverted FS |
- Inverted staff = negative reading in the HI formula.
- HI1 = 101.50 + (−2.25) = 99.25 m; HI2 = 98.20 + 1.35 = 99.55 m.
- RL(C) = 99.55 − (−4.100) = 103.65 m.