PS Surveying Equipment & Technology 3 — Questions and Answers
Question 1: What is the function of the 'circle zero set' procedure performed before taking angular measurements with a theodolite?
- Levels the instrument precisely
- Orients the horizontal circle to a desired initial direction (Correct answer)
- Resets the vertical index error to zero
- Calibrates the EDM prism offset
Correct answer: Orients the horizontal circle to a desired initial direction
Circle zero set (or circle set) positions the horizontal circle so that a reference backsight direction reads a predetermined value, typically 0°00'00".
Question 2: In static GNSS baseline processing, what is the significance of 'fixing the integer ambiguity'?
- Setting the ellipsoid height datum to a fixed value
- Resolving the unknown whole number of carrier wave cycles between satellite and receiver to achieve centimeter accuracy (Correct answer)
- Locking the base receiver coordinates to the control point
- Fixing the antenna phase center to the measurement mark
Correct answer: Resolving the unknown whole number of carrier wave cycles between satellite and receiver to achieve centimeter accuracy
Fixing integer ambiguities allows use of the precise carrier phase measurement rather than pseudo-range, yielding sub-centimeter baseline precision.
Question 3: Which instrument is best suited for establishing precise vertical control along a long highway corridor where differential leveling would be impractical?
- Trigonometric leveling with a total station using long sights
- Digital level with invar rod
- Network RTK GNSS with accurate geoid model (Correct answer)
- Hand level with stadia rod
Correct answer: Network RTK GNSS with accurate geoid model
Network RTK GNSS combined with a high-accuracy geoid model (e.g., GEOID18) can provide rapid orthometric heights accurate to 2–3 cm over long corridors.
Question 4: What is the primary drawback of using a 360° prism (omnidirectional prism) with a robotic total station?
- It cannot be used indoors
- Its effective prism constant varies with the angle of incidence, introducing distance errors (Correct answer)
- It only works at distances under 100 m
- It requires a different EDM frequency than standard prisms
Correct answer: Its effective prism constant varies with the angle of incidence, introducing distance errors
Because the signal enters a 360° prism from varying angles as the target moves, the effective prism constant changes slightly, introducing small but real distance errors if not properly modeled.
Question 5: A terrestrial laser scanner reports a range noise of ±2 mm at 50 m. What is the primary source of this random range error?
- Clock synchronization between scanner and target
- Signal-to-noise ratio in the detector circuit and surface reflectivity variation (Correct answer)
- Atmospheric refraction at close range
- Mirror wobble in the rotating scan head
Correct answer: Signal-to-noise ratio in the detector circuit and surface reflectivity variation
Range noise in TLS systems is dominated by detector noise and target reflectivity; darker or angled surfaces return fewer photons, increasing measurement uncertainty.
Question 6: When performing a two-peg test on an automatic level, what condition indicates the instrument is in adjustment?
- The rod readings are equal at both pegs
- The difference in rod readings from both setups is the same, confirming the line of sight is truly horizontal (Correct answer)
- The bubble remains centered throughout the test
- The compensator deflects equally in both directions
Correct answer: The difference in rod readings from both setups is the same, confirming the line of sight is truly horizontal
The two-peg test compares height differences from two instrument positions; if both give the same Δh, the collimation error is zero and the line of sight is horizontal.
Question 7: Which GNSS error is effectively eliminated by differencing observations between two receivers simultaneously tracking the same satellites?
- Receiver clock error only
- Satellite clock error and satellite clock error simultaneously with receiver hardware delay
- Both satellite clock errors and common atmospheric errors (Correct answer)
- Multipath and receiver noise
Correct answer: Both satellite clock errors and common atmospheric errors
Single differencing between receivers on the same baselines eliminates satellite clock errors and largely cancels ionospheric and tropospheric delays shared by both receivers.
PS Surveying Equipment & Technology 3
What is the function of the 'circle zero set' procedure performed before taking angular measurements with a theodolite?