In levelling, a foresight (FS) is defined as:
Correct Answer: A. The staff reading taken at the change point before the instrument is shifted
📚 Detailed Explanation: Foresight Is the Last Reading Before the Instrument Moves
Why A (Staff reading taken at change point before the instrument is shifted) is correct: A foresight (FS) is the last staff reading taken from a given instrument position. It is taken on a change point (CP) or on the closing benchmark, and this same point then receives the next backsight reading after the instrument has been moved forward.
Reading Types in Levelling
| Reading | Position in Sequence | Taken on | Purpose |
|---|---|---|---|
| Back Sight (BS) | First reading at each instrument position | BM or CP (from previous setup) | Establishes HI for that setup |
| Intermediate Sight (IS) | Any reading between BS and FS | Any intermediate point whose RL is needed | Calculate RL from HI |
| Fore Sight (FS) | Last reading at each instrument position; taken before instrument moves | Change point (CP) or closing BM | Establishes RL of the next CP; closes current HI |
Change Point (CP): The CP receives both a FS (from the current instrument position) and then a BS (from the next instrument position). The CP 'links' successive instrument setups. The staff is held firmly on the CP while the instrument moves.
- Foresight = last reading at a setup, taken on the next change point or closing BM.
- The instrument moves after the FS is booked; the next setup begins with a BS on the same CP.
- ΣBS − ΣFS = Last RL − First RL (arithmetic check).