Levelling along a falling gradient with a 3 m staff. Successive readings: 1.155, 2.74, 0.75, 1.79 m. Level shifted after 2nd reading. Correct booking order is:
Correct Answer: A. BS, FS, BS, FS
📚 Detailed Explanation: Falling Gradient Readings — Correct Booking Order = BS, FS, BS, FS
Why A (BS, FS, BS, FS) is correct: On a falling slope with a 3 m staff, when the staff reading approaches the staff's maximum length, the instrument must be relocated before taking another reading. In this case, the second reading (2.74 m) is close to the 3 m limit, so the instrument is shifted and the second reading becomes a Foresight. The third reading resets to 0.75 m (new BS after shift), and the fourth is the final FS.
Readings: 1.155 m → 2.74 m → 0.75 m → 1.79 m
Staff length: 3 m (maximum readable)
Ground: falling gradient
Staff length: 3 m (maximum readable)
Ground: falling gradient
Analysis:
1.155 m: First reading at start → Back Sight (BS)
2.74 m: Ground has dropped; reading jumped near staff limit (3 m).
Instrument must be shifted AFTER this reading → Fore Sight (FS)
0.75 m: Instrument was shifted; this is first reading at new setup → Back Sight (BS)
1.79 m: Final reading before next shift or end → Fore Sight (FS)
Booking order: BS, FS, BS, FS ✓
Why There Are No Intermediate Sights
| Reading | Value (m) | Type | Reason |
|---|---|---|---|
| 1st | 1.155 | BS | First reading after setup; known point BM |
| 2nd | 2.74 | FS | Steep fall forced an instrument shift immediately; no IS taken |
| 3rd | 0.75 | BS | First reading at new setup; change point is now the BM |
| 4th | 1.79 | FS | Final reading; another shift will be needed |
- When a reading approaches the staff limit and the ground is still falling, the instrument must shift immediately → that reading becomes a FS.
- Correct booking: BS, FS, BS, FS.
- On a falling gradient, staff readings keep increasing — they can hit the 3 m limit quickly.