FUNDAMENTAL SURVEYING Surveying Equipment & GPS/GNSS 2 — Questions and Answers
Question 1: What is 'DOP' (Dilution of Precision) in GPS surveying?
- The percentage of time a GPS receiver is operational
- A dimensionless number indicating the geometric strength of the satellite configuration (Correct answer)
- The signal delay caused by the ionosphere
- The number of satellites in a constellation
Correct answer: A dimensionless number indicating the geometric strength of the satellite configuration
DOP values indicate how satellite geometry affects position accuracy — a low PDOP (such as less than 4) means good geometry and better accuracy.
Question 2: Which type of GPS error is largely eliminated by differential techniques (DGPS or RTK)?
- Antenna phase center variation
- Common-mode atmospheric errors that affect both base and rover (Correct answer)
- Multipath from nearby reflective surfaces
- Receiver clock drift in the rover only
Correct answer: Common-mode atmospheric errors that affect both base and rover
Differential GPS cancels errors common to both the base and rover (atmospheric delays, clock biases) as they cancel in the differencing process.
Question 3: What is 'geodetic height' (ellipsoidal height) in GPS terminology?
- The elevation above mean sea level (orthometric height)
- The height above the reference ellipsoid (GRS 80 or WGS 84) (Correct answer)
- The height above the local geoid only
- The height corrected for atmospheric refraction
Correct answer: The height above the reference ellipsoid (GRS 80 or WGS 84)
Geodetic (ellipsoidal) height is the distance above the mathematical reference ellipsoid, not sea level; it must be corrected by the geoid undulation to get orthometric height.
Question 4: What is a 'tribrach' in surveying instrumentation?
- A three-legged rod used to hold a prism over a point
- A leveling base with three foot screws that mounts instruments on a tripod head (Correct answer)
- A three-crosshair reticle used in precise leveling
- A type of tripod with only three sections
Correct answer: A leveling base with three foot screws that mounts instruments on a tripod head
A tribrach is a leveling base with three leveling screws and a forced-centering plate that attaches to a tripod and accepts interchangeable survey instruments.
Question 5: What is 'forced centering' in surveying?
- A method to force-close a traverse with excess error
- A system allowing interchangeable instruments to be swapped onto a tribrach without re-centering over the point (Correct answer)
- The process of centering a level bubble electronically
- A GPS initialization method for RTK surveys
Correct answer: A system allowing interchangeable instruments to be swapped onto a tribrach without re-centering over the point
Forced centering uses matched tribrachs and adapters so that a total station, prism, or target can be interchanged at the same setup point without disturbing the centered, leveled base.
Question 6: What is the purpose of atmospheric corrections applied to EDM measurements?
- To correct for the curvature of the Earth along the measurement path
- To adjust the velocity of electromagnetic waves for actual air temperature, pressure, and humidity (Correct answer)
- To compensate for the magnetic effect of the atmosphere on the prism
- To reduce slope distance to horizontal distance
Correct answer: To adjust the velocity of electromagnetic waves for actual air temperature, pressure, and humidity
EDMs assume a standard atmosphere; corrections for actual temperature, pressure, and humidity adjust the wave propagation speed to give the true distance.
FUNDAMENTAL SURVEYING Surveying Equipment & GPS/GNSS 2
What is 'DOP' (Dilution of Precision) in GPS surveying?