Which of the following is an INCORRECT statement related to the height of instrument (HI) used in levelling?
Correct Answer: D. HI never changes after recording back sights
📚 Detailed Explanation: INCORRECT HI Statement — HI Does NOT Remain Constant After Every BS
This question asks for the INCORRECT statement about Height of Instrument (HI). Statement D says “The value of HI never changes after recording back sights” — this is false because whenever the instrument is moved to a new position and a new BS is taken, a completely new HI is established.
What Is HI and When Does It Change?
| Statement | Verdict | Explanation |
|---|---|---|
| A. HI never changes while recording fore sights | ✓ TRUE | Within one instrument setup, the HI is fixed while all FS readings are taken before the instrument moves |
| B. HI generally changes before recording back sights | ✓ TRUE | The instrument is moved to a new location; the new HI is established by the new BS on a change point or BM |
| C. HI never changes while recording intermediate sights | ✓ TRUE | All IS readings at a single setup use the same HI value |
| D. HI never changes after recording back sights | ✗ INCORRECT | FALSE. The HI established by one BS remains fixed ONLY until the instrument moves. When the instrument relocates and takes a NEW BS, a completely new HI is calculated. HI CAN change multiple times throughout a levelling traverse. |
Example — HI changes at each new instrument setup:
Setup 1: HI1 = RL(BM) + BS1 = 100 + 1.5 = 101.5 m
…take IS and FS readings…
Setup 2: HI2 = RL(CP) + BS2 = 99.5 + 1.8 = 101.3 m ← NEW HI (changed!)
…take more IS and FS readings…
Setup 3: HI3 = RL(CP2) + BS3 = 101.0 + 2.2 = 103.2 m ← CHANGED AGAIN!
Setup 1: HI1 = RL(BM) + BS1 = 100 + 1.5 = 101.5 m
…take IS and FS readings…
Setup 2: HI2 = RL(CP) + BS2 = 99.5 + 1.8 = 101.3 m ← NEW HI (changed!)
…take more IS and FS readings…
Setup 3: HI3 = RL(CP2) + BS3 = 101.0 + 2.2 = 103.2 m ← CHANGED AGAIN!
- HI is constant within a single instrument setup for all IS and FS readings.
- HI changes every time the instrument is relocated and a new BS is taken.
- Statement D is INCORRECT: the phrase “never changes after recording back sights” wrongly implies permanence.