GISP GPS and Field Data Collection 1 — Questions and Answers
Question 1: What is the minimum number of GPS satellites required to calculate a 3D position fix (latitude, longitude, and elevation)?
- 2
- 3
- 4 (Correct answer)
- 6
Correct answer: 4
A minimum of four satellites is required for a 3D GPS fix — three to solve for the three position coordinates and a fourth to eliminate clock error in the receiver.
Question 2: Differential GPS (DGPS) improves positional accuracy by:
- Using more satellite signals simultaneously
- Applying correction signals from a stationary base station at a known location (Correct answer)
- Processing data in dual-frequency mode
- Averaging multiple position readings over time
Correct answer: Applying correction signals from a stationary base station at a known location
DGPS uses a fixed reference receiver at a precisely surveyed location to compute positional errors and broadcast real-time corrections to roving GPS receivers in the field.
Question 3: Which GPS error source results from signals bouncing off nearby buildings, vehicles, or terrain before reaching the receiver antenna?
- Ionospheric delay
- Multipath error (Correct answer)
- Ephemeris error
- Selective Availability
Correct answer: Multipath error
Multipath error occurs when GPS signals reflect off nearby surfaces and travel longer indirect paths to the receiver, causing inaccurate pseudorange measurements.
Question 4: What does Dilution of Precision (DOP) measure in GPS surveys?
- The signal strength of individual satellites
- The geometric quality of available satellite configuration affecting positional accuracy (Correct answer)
- The time required to achieve a satellite fix
- The amount of atmospheric signal delay
Correct answer: The geometric quality of available satellite configuration affecting positional accuracy
DOP quantifies how satellite geometry affects positional accuracy — a low DOP value (closer to 1) indicates favorable satellite spread and more precise positioning.
Question 5: Real-Time Kinematic (RTK) GPS achieves centimeter-level accuracy by:
- Averaging hundreds of position readings over several hours
- Using carrier phase measurements and a real-time base station correction link (Correct answer)
- Applying post-processed ionospheric models
- Selecting only satellites above 45-degree elevation
Correct answer: Using carrier phase measurements and a real-time base station correction link
RTK GPS resolves integer ambiguities in carrier phase measurements using real-time corrections from a base station, enabling centimeter-level accuracy in the field.
Question 6: In GIS field data collection, which file format is the native exchange format for GNSS waypoints, routes, and tracks commonly supported by handheld GPS devices?
- Shapefile (.shp)
- GPX (GPS Exchange Format) (Correct answer)
- KMZ (Keyhole Markup, compressed)
- GeoJSON
Correct answer: GPX (GPS Exchange Format)
GPX is an open XML-based format designed specifically for GPS data — it stores waypoints, tracks, and routes and is supported by virtually all GPS devices and many GIS applications.
GISP GPS and Field Data Collection 1
What is the minimum number of GPS satellites required to calculate a 3D position fix (latitude, longitude, and elevation)?