A line of levels (a levelling run) in surveying commences with:
Correct Answer: C. A back sight and closes with a fore sight
📚 Detailed Explanation: A Line of Levels Commences with BS and Closes with FS
Why C (Commences with a back sight and closes with a fore sight) is correct: By definition and convention in levelling field books, the first reading taken at a new instrument position is on a point of known (or assumed) RL. This reading is the Back Sight (BS). The last reading taken from that position (or the final reading of a traverse) is on the closing bench mark or closing point. This reading is the Fore Sight (FS).
Levelling Field Book Conventions
| Reading Type | When Taken | Effect |
|---|---|---|
| Back Sight (BS) | First reading at each instrument setup; taken on a point of known RL (BM or CP) | Establishes the Height of Instrument (HI = RL + BS) |
| Intermediate Sight (IS) | Any reading taken between the BS and the FS from the same setup | Used to calculate RL of intermediate points; RL = HI − IS |
| Fore Sight (FS) | Last reading at each instrument setup; taken on the next change point or on the closing BM | Establishes RL of the next point (CP); closes the current setup |
A typical levelling run:
BS (at BM) → IS … IS … IS → FS (at CP1)
Move instrument
BS (at CP1) → IS … IS … IS → FS (at CP2)
…
BS (at CP_n) → FS (at closing BM)
BS (at BM) → IS … IS … IS → FS (at CP1)
Move instrument
BS (at CP1) → IS … IS … IS → FS (at CP2)
…
BS (at CP_n) → FS (at closing BM)
First entry in field book: BS
Last entry in field book: FS
- Every levelling run commences with a BS and closes with a FS.
- Arithmetic check: ΣBS − ΣFS = Last RL − First RL.
- Only BS and FS readings appear in the check; IS readings do not affect the ΣBS − ΣFS check.