PS Traverse & Leveling Calculations 3 — Questions and Answers
Question 1: In differential leveling, a backsight (BS) of 8.42 ft is taken on BM-A with elevation 312.65 ft. What is the Height of Instrument (HI)?
- 321.07 ft (Correct answer)
- 304.23 ft
- 316.07 ft
- 308.65 ft
Correct answer: 321.07 ft
HI = Elevation of BM + BS reading = 312.65 + 8.42 = 321.07 ft.
Question 2: With an HI of 321.07 ft and a foresight (FS) of 5.63 ft on TP-1, what is the elevation of TP-1?
- 315.44 ft (Correct answer)
- 326.70 ft
- 310.19 ft
- 321.07 ft
Correct answer: 315.44 ft
Elevation of TP-1 = HI − FS = 321.07 − 5.63 = 315.44 ft.
Question 3: What arithmetic check verifies differential leveling notes for a closed loop returning to the starting benchmark?
- ΣBS − ΣFS should equal zero (Correct answer)
- ΣBS + ΣFS should equal the total loop distance
- The count of BS readings must exceed FS readings by one
- Each turning-point elevation must be a whole-foot value
Correct answer: ΣBS − ΣFS should equal zero
For a closed leveling loop, ΣBS − ΣFS = 0 confirms arithmetic; for an open line it equals the net elevation change.
Question 4: Three-wire leveling improves precision by:
- Reading top, middle, and bottom stadia crosshairs to average the rod reading and detect errors (Correct answer)
- Using three separate leveling rods at each setup
- Taking three independent measurements and averaging the HI
- Applying three independent benchmark checks on each circuit
Correct answer: Reading top, middle, and bottom stadia crosshairs to average the rod reading and detect errors
Three-wire leveling reads all three horizontal crosshairs; the mean of the top and bottom wires should match the center wire, providing a check.
Question 5: Reciprocal leveling is primarily used to:
- Carry elevations across wide obstacles such as rivers where a balanced setup is impossible (Correct answer)
- Eliminate the need for turning points on long lines
- Reduce the total number of backsight readings required
- Check the collimation error of the level instrument
Correct answer: Carry elevations across wide obstacles such as rivers where a balanced setup is impossible
Reciprocal leveling makes setups on both sides of an obstacle and averages results, largely canceling curvature and refraction errors.
Question 6: The combined effect of Earth curvature and atmospheric refraction on a sight distance of 1 mile is approximately:
- 0.57 ft (Correct answer)
- 0.21 ft
- 1.33 ft
- 0.09 ft
Correct answer: 0.57 ft
Combined curvature and refraction correction ≈ 0.574 M² ft (M in miles), giving about 0.57 ft at 1 mile.
Question 7: In profile leveling, intermediate foresights (IFS) differ from turning-point foresights (FS) in that:
- IFS readings yield ground elevations but do not establish a new HI — the instrument is not moved (Correct answer)
- IFS readings require computing a new HI before continuing
- IFS and FS readings serve identical computational purposes
- IFS readings are taken only at whole-station intervals along the alignment
Correct answer: IFS readings yield ground elevations but do not establish a new HI — the instrument is not moved
An intermediate foresight is a minus-sight only (Elev = HI − IFS); no turning point is established, so the HI remains unchanged.
PS Traverse & Leveling Calculations 3
In differential leveling, a backsight (BS) of 8.42 ft is taken on BM-A with elevation 312.65 ft.
What is the Height of Instrument (HI)?