For which of the following applications in levelling is an inverted staff reading most suitable?
Correct Answer: A. Levelling across a wall
📚 Detailed Explanation: Inverted Staff Is Used for Levelling Across a Wall (Overhead Structures)
Why A (levelling across a wall) is correct: An inverted staff reading is used when the target point lies above the horizontal line of sight — such as the underside of a lintel over a doorway, the soffit of a bridge, the bottom of a beam, or the underside of a slab. The staff is held upside down against the overhead surface, and the reading is treated as negative in calculations.
When to Use Inverted Staff Reading
| Situation | Inverted Staff? | Why |
|---|---|---|
| Underside of lintel above a wall opening | ✓ YES | Target is above line of sight; staff cannot be rested on top |
| Soffit of bridge deck | ✓ YES | Bridge deck underside is above instrument line of sight |
| Roof/ceiling of a room | ✓ YES | Ceiling is above line of sight; standard staff cannot reach it upright |
| High or low intervening ground | ✗ NO | Ground features do not require inverted staff; reciprocal levelling or differential levelling is used |
| Levelling across a lake | ✗ NO | Reciprocal levelling or special arrangements are used |
| Steep slope | ✗ NO | Staff held upright on slope points; instrument setup changed more frequently |
Key Rule: Inverted staff reading (positive measured value) is treated as negative when computing HI or RL. If BM is overhead: HI = RL(BM) + (−reading); RL(target) = HI − (−reading) = HI + reading.
- Inverted staff = target is above the instrument's line of sight (overhead structure).
- Most common use: underside of a wall lintel, bridge soffit, ceiling, beam.
- Reading is entered as negative in HI and RL calculations.