Which of the following staff readings are taken at a turning point (change point) that necessitates an instrument change in levelling?
Correct Answer: D. Both A (Foresight) and B (Backsight)
📚 Detailed Explanation: At a Turning Point, Both Foresight AND Backsight Are Required
Why D (Both A and B — Foresight and Backsight) is correct: A turning point (change point, CP) is a temporary point where the instrument is relocated. Two readings must be taken at the same physical point: a Foresight (FS) from the old instrument position (to record the CP’s elevation) and then a Backsight (BS) from the new instrument position (to establish the new HI). The staff stays at the CP while the instrument moves.
Sequence at a Change Point
| Step | Reading Type | Staff Position | Instrument Position | Purpose |
|---|---|---|---|---|
| 1 | Foresight (FS) | At CP | Old setup (Setup 1) | Determine RL of the CP from the current HI |
| 2 | (Instrument moved) | Staff remains at CP | Moving to new location | Instrument relocated; staff stays put |
| 3 | Backsight (BS) | At CP (same point) | New setup (Setup 2) | Establish new HI using the known RL of the CP |
Why Not Intermediate Sight (C) at a CP?
| Reading | At Change Point? | Reason |
|---|---|---|
| Foresight (A) | ✓ YES — mandatory | Last reading from old setup; determines CP elevation |
| Backsight (B) | ✓ YES — mandatory | First reading from new setup; establishes new HI |
| Intermediate sight (C) | ✗ NO | IS is taken at intermediate stations between BS and FS; never at a CP where the instrument must move |
- At a change point: Foresight (FS) from old setup + Backsight (BS) from new setup — both mandatory.
- The staff stays at the CP while the instrument is relocated; this ensures continuity of the levelling chain.
- Intermediate sights are never taken at change points.