In levelling, which of the following is also called a ‘minus sight’?
Correct Answer: B. Fore sight
📚 Detailed Explanation: Fore Sight (FS) is Also Called Minus Sight
Why B (Fore sight) is correct: The fore sight is the staff reading taken on a point whose elevation needs to be determined. It is subtracted from the Height of Instrument (HI) to compute the Reduced Level: RL = HI − FS. Because it is subtracted, it is called the minus sight. Conversely, the back sight is added to the RL to compute HI, so it is called the plus sight.
Comparison of Staff Reading Types
| Reading Type | When Taken | Used To | Also Called |
|---|---|---|---|
| Back Sight (BS) | First reading after setting up instrument; taken on a point of known RL | Compute Height of Instrument: HI = RL + BS | Plus sight (added) |
| Fore Sight (FS) | Last reading before shifting instrument; on point whose RL is to be found | Compute RL of new point: RL = HI − FS | Minus sight (subtracted) |
| Intermediate Sight (IS) | Any reading between BS and FS at same instrument setup | Compute RL: RL = HI − IS | Neither plus nor minus sight |
HI method summary:
HI = RL(known) + BS ← BS is the PLUS sight
RL = HI – FS ← FS is the MINUS sight
RL = HI – IS ← IS also subtracted (like FS)
HI = RL(known) + BS ← BS is the PLUS sight
RL = HI – FS ← FS is the MINUS sight
RL = HI – IS ← IS also subtracted (like FS)
- Fore sight (FS) = minus sight — it is subtracted from HI to find the new RL.
- Back sight (BS) = plus sight — it is added to the known RL to find HI.
- Intermediate sight (IS) is also subtracted from HI but is not at a change point.