In a levelling survey, the first staff reading taken after shifting the instrument and the last staff reading taken before the instrument is shifted are, respectively:
Correct Answer: C. back sight and fore sight
📚 Detailed Explanation: First Reading After Shift = Back Sight; Last Before Shift = Fore Sight
Why C (back sight and fore sight) is correct: In any levelling traverse, the sequence is strict: the FIRST staff reading taken after the instrument is set up at a new location is always a Back Sight (BS) — on a known point (BM or CP) to establish HI. The LAST reading before the instrument is moved is always a Fore Sight (FS) — on the new change point to establish its RL before shifting.
Standard Sequence in a Levelling Traverse
| Reading Type | When Taken | Reading Order | Purpose |
|---|---|---|---|
| Back Sight (BS) | First reading after instrument setup | 1st at each setup | Establish new HI = RL(known) + BS |
| Intermediate Sight (IS) | Any reading between BS and FS at same setup | 2nd, 3rd, etc. | Find RL of intermediate points: RL = HI − IS |
| Fore Sight (FS) | Last reading before instrument is moved | Last at each setup | Find RL of next CP: RL = HI − FS |
Why the Other Options Are Wrong
| Option | Claim | Why Wrong |
|---|---|---|
| A | BS and IS | Intermediate sight is NOT the last reading before shifting; FS is. |
| B | IS and BS | Inverted; first reading is BS, not IS. |
| C | BS and FS | CORRECT — first after shift = BS; last before shift = FS |
| D | FS and BS | Inverted order; FS comes last (before shift), BS comes first (after shift). |
- First reading after instrument shift: Back Sight (BS) on a known/change point.
- Last reading before instrument shift: Fore Sight (FS) on the next change point.
- Levelling line always: starts with BS on BM → ends with FS on final station.