Which of the following is an INCORRECT statement related to the height of the instrument (HI) used in levelling?
- AThe value of HI generally changes before recording back sights.
- BThe value of HI never changes after recording back sights.Correct
- CThe value of HI never changes while recording intermediate sights.
- DThe value of HI never changes while recording fore sights.
Solution
Concept
HI = RL of point + back sight. HI stays the same for one instrument setup and changes only when the instrument is shifted.
Formula
HI = RL of BM + BS; RL of point = HI - (IS or FS)
Step-by-step solution
1. HI is fixed by the back sight taken on a point of known RL: HI = RL + BS. 2. At the same setup, IS and FS are taken with this same HI, so HI does not change while recording IS or FS. 3. After the FS on a change point, the instrument is shifted, so HI changes. 4. A new HI is then fixed by a new BS. So HI changes before a new BS is recorded. Statement A is correct. 5. Statement B says HI never changes after recording a BS. This is wrong, because HI changes when the instrument is shifted after the FS. 6. So the incorrect statement is B.
Explanation
HI depends only on the instrument setup. It stays constant while IS and FS are read, and it changes when the instrument is moved. Each move happens after a fore sight and before the next back sight, so saying HI 'never' changes after a back sight is incorrect.
Why the other options are wrong
A: Correct. HI changes when the instrument is shifted, before the new BS. C: Correct. HI is constant while IS are read. D: Correct. HI is constant while the FS is read at that setup.
Shortcut
HI changes only when the instrument moves, that is between FS and the next BS.
Common mistake
Thinking a BS reading itself fixes HI forever. It fixes HI only for that one setup.
Correct answer: B) The value of HI never changes after recording back sights.