The RL of point A is 100 m and its back sight is 2.00 m. The next reading at the change point is 1.5 m. Find the RL of the change point.
Correct Answer: B. 100.5 m
📚 Detailed Explanation: RL of Change Point = HI − FS = 100.5 m
Why B (100.5 m) is correct: The BS at A establishes the Height of Instrument. The reading at the change point is a Foresight (FS), subtracted from HI to find the RL of the change point.
Given:
RL of Point A = 100 m
Back Sight (BS) at A = 2.00 m
Reading at Change Point = 1.5 m (this is a Foresight, FS)
RL of Point A = 100 m
Back Sight (BS) at A = 2.00 m
Reading at Change Point = 1.5 m (this is a Foresight, FS)
Calculation:
HI = RL(A) + BS = 100 + 2.00 = 102 m
RL of Change Point = HI – FS = 102 – 1.5 = 100.5 m
Role of the Change Point
| Reading Type | At Change Point | Purpose |
|---|---|---|
| FS = 1.5 m (taken from current setup) | Determines RL of CP = 100.5 m | Ends the current levelling run before instrument shift |
| BS (taken after instrument shift) | Establishes new HI from RL(CP) = 100.5 m | Begins the next levelling leg |
Why Not Option C (102 m)?
Option C gives 102 m, which is the HI, not the RL of the change point.
HI = RL(A) + BS = 102 m ← this is the HEIGHT OF INSTRUMENT, not the RL of CP
RL(CP) = HI – FS = 102 – 1.5 = 100.5 m ← this is the correct answer
HI = RL(A) + BS = 102 m ← this is the HEIGHT OF INSTRUMENT, not the RL of CP
RL(CP) = HI – FS = 102 – 1.5 = 100.5 m ← this is the correct answer
- HI = RL(A) + BS = 100 + 2.00 = 102 m.
- RL(CP) = HI − FS = 102 − 1.5 = 100.5 m.
- The reading at the change point is always a Foresight (FS), not a BS or IS.