A levelling traverse from A to C with one change point B uses staff readings in what order?
Correct Answer: C. BS, FS, BS, FS
📚 Detailed Explanation: Correct Sequence of Staff Readings = BS, FS, BS, FS
Why C (BS, FS, BS, FS) is correct: In a standard levelling traverse with one change point (CP), the sequence of staff readings is: Back Sight (BS) at the BM, Fore Sight (FS) at the change point, instrument moves, Back Sight (BS) on the same CP, and Fore Sight (FS) at the closing BM. This gives the pattern BS → FS → BS → FS.
Standard 4-reading levelling run with one change point:
Reading 1: BS at BM (starting benchmark) → establishes HI₁
Reading 2: FS at CP (change point) → closes HI₁; gives RL(CP)
(Instrument moved to between CP and closing BM)
Reading 3: BS at CP (same change point) → establishes HI₂
Reading 4: FS at closing BM or final point → closes HI₂; gives RL(end)
Reading 1: BS at BM (starting benchmark) → establishes HI₁
Reading 2: FS at CP (change point) → closes HI₁; gives RL(CP)
(Instrument moved to between CP and closing BM)
Reading 3: BS at CP (same change point) → establishes HI₂
Reading 4: FS at closing BM or final point → closes HI₂; gives RL(end)
Pattern: BS → FS → BS → FS (option C)
Why This Pattern Is Necessary
| Reading | Purpose | Requires Instrument Position |
|---|---|---|
| 1st BS (at BM) | Start the levelling run; establish HI from known RL | Setup 1 (between BM and CP) |
| 1st FS (at CP) | End Setup 1; record RL of change point | Setup 1 |
| 2nd BS (at CP) | Start Setup 2; re-establish HI from now-known RL(CP) | Setup 2 (between CP and end) |
| 2nd FS (at end) | End Setup 2; record RL of final point | Setup 2 |
- Standard pattern for 1-CP levelling run: BS, FS, BS, FS.
- The same CP receives the 1st FS and then the 2nd BS — it is the link between the two setups.
- Every additional change point adds another BS+FS pair to the sequence.