FUNDAMENTAL SURVEYING Leveling & Differential Leveling 2 — Questions and Answers
Question 1: What is the allowable closure for third-order leveling per NGS specifications (in feet, where M = miles)?
- 0.010 sqrt M
- 0.050 sqrt M
- 0.035 sqrt M (Correct answer)
- 0.100 sqrt M
Correct answer: 0.035 sqrt M
NGS third-order leveling allows a closure of 0.035 sqrt(M) feet, where M is the total distance leveled in miles.
Question 2: Curvature and refraction combined correction (C&R) in leveling is approximately how many feet per 1,000 ft of sight distance?
- 0.0239 ft (Correct answer)
- 0.0574 ft
- 0.0021 ft
- 0.1120 ft
Correct answer: 0.0239 ft
The combined curvature and refraction correction is approximately 0.0239F squared feet where F is sight distance in thousands of feet, about 0.024 ft per 1,000 ft.
Question 3: What is the purpose of balancing backsight and foresight distances in differential leveling?
- To reduce instrument setup time
- To cancel out errors from instrument mis-leveling and curvature/refraction (Correct answer)
- To ensure the rod is read at the same time of day
- To keep the traverse closed
Correct answer: To cancel out errors from instrument mis-leveling and curvature/refraction
Equal BS and FS distances ensure that any small collimation error and curvature/refraction effects cancel out, as they affect both readings equally.
Question 4: A level loop starts and ends at BM-A (elevation 100.000 ft). The computed closing elevation is 100.042 ft. What is the closure error?
- +0.042 ft (Correct answer)
- -0.042 ft
- 0.000 ft
- 100.042 ft
Correct answer: +0.042 ft
Closure error = computed final elevation minus known starting elevation = 100.042 minus 100.000 = +0.042 ft.
Question 5: What is 'reciprocal leveling' used for?
- Leveling between two benchmarks at equal elevations
- Accurately determining elevation differences across wide obstacles like rivers (Correct answer)
- Calibrating a level rod against a benchmark
- Measuring elevations with a hand level and Abney level
Correct answer: Accurately determining elevation differences across wide obstacles like rivers
Reciprocal leveling averages observations taken from both sides of an obstacle (e.g., a river) to eliminate curvature, refraction, and instrument errors.
Question 6: In a level notes table, the foresight rod reading is subtracted from HI to get:
- The backsight elevation
- The elevation of the foresight point (Correct answer)
- The instrument height above ground
- The total closure error
Correct answer: The elevation of the foresight point
Point elevation = HI minus foresight rod reading.
FUNDAMENTAL SURVEYING Leveling & Differential Leveling 2
What is the allowable closure for third-order leveling per NGS specifications (in feet, where M = miles)?