Original question written by FE Exam AI Prep from the public FE Civil exam specification. Before release, a solver model re-derived the answer twice from the question alone, without its answer key, and a critic model checked the result, and the arithmetic was re-run as an executable calculation. No licensed engineer reviews these questions. It is not an NCEES question and does not come from any NCEES practice exam. FE Exam AI Prep is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCEES.
Differential Leveling and Height of Instrument: FE Civil practice problem 4
For the West Basin airport access road, a project engineer preparing an operations handoff review runs a level loop from benchmark elevation 518.75 ft with backsight 4.46 ft and foresight 4.26 ft. What turning point elevation is computed?
- A326.69 ft
- B578.63 ft
- C518.95 ft
- D645.53 ft
Show the answer and worked solution
Answer: C — 518.95 ft
Worked solution
- Step 1. Compute height of instrument
HI = BM + BS
HI = 518.75 + 4.46 = 523.21 ft
- Step 2. Subtract foresight
elevation = HI - FS
elevation = 523.21 - 4.26 = 518.95 ft
Why the other choices are wrong
- Choice A
- This figure lands close to seventy-eight percent of the correct turning point elevation, far below anything a single backsight or foresight substitution in HI = benchmark + backsight would leave behind; the sound path adds the backsight to the benchmark for the height of instrument, then subtracts the foresight from that sum.
- Choice B
- This figure sits roughly forty-five percent above the correct turning point elevation, far beyond what a single backsight or foresight slip could produce in the benchmark-plus-backsight-minus-foresight sequence; form the height of instrument from benchmark plus backsight, then subtract the foresight to get the answer.
- Choice D
- This figure runs about eighteen percent above the correct turning point elevation, a gap too large to trace to one dropped or doubled term in the height-of-instrument arithmetic; recompute by summing benchmark and backsight for HI, then subtracting the foresight to reach the true elevation.