GISP GPS and Field Data Collection 2 — Questions and Answers
Question 1: What is the primary purpose of a control point survey in establishing a geodetic network for GIS mapping projects?
- To calibrate remote sensing imagery color balance
- To provide precisely measured reference locations for georeferencing and spatial accuracy (Correct answer)
- To record attribute data for database population
- To delineate property boundaries for cadastral mapping
Correct answer: To provide precisely measured reference locations for georeferencing and spatial accuracy
Control point surveys establish accurately positioned reference monuments that serve as the foundation for georeferencing maps, imagery, and other spatial data to a consistent coordinate framework.
Question 2: When collecting GPS points for a polygon boundary, what field practice minimizes positional error at vertices?
- Recording a single instant reading at each vertex
- Averaging multiple position readings (position averaging) at each vertex (Correct answer)
- Using the first satellite lock acquired
- Collecting points only during midday hours
Correct answer: Averaging multiple position readings (position averaging) at each vertex
Position averaging records multiple GPS readings over a period of time at each vertex and averages them, reducing random errors and improving the accuracy of boundary vertex locations.
Question 3: Which datum transformation parameter set is required when converting GPS coordinates (WGS84) to NAD83 for use in US federal mapping projects?
- NADCON or HARN grid-based transformation (Correct answer)
- Simple 3-parameter Molodensky transformation
- Helmert 7-parameter transformation only
- No transformation is needed as WGS84 and NAD83 are identical
Correct answer: NADCON or HARN grid-based transformation
NADCON (North American Datum Conversion) and HARN (High Accuracy Reference Networks) provide grid-shift files that accurately transform coordinates between WGS84 and NAD83, required for precise federal geospatial work.
Question 4: In a GIS field data collection workflow, what is the primary advantage of using a geodatabase schema loaded onto a mobile device over collecting raw GPS tracks?
- Raw GPS tracks have higher spatial accuracy
- Attribute data is collected simultaneously with geometry using validated domain fields (Correct answer)
- Mobile geodatabases can acquire more satellite signals
- GPS tracks do not require post-processing
Correct answer: Attribute data is collected simultaneously with geometry using validated domain fields
A pre-loaded geodatabase schema with coded value domains, default values, and required fields ensures that attribute data is captured accurately alongside geometry during the field collection process.
Question 5: What is the significance of the PDOP (Position Dilution of Precision) threshold of 6 commonly used in GPS field surveys?
- It is the minimum number of satellites required for a fix
- It is the maximum acceptable geometric dilution before positional accuracy becomes unreliable for most GIS applications (Correct answer)
- It represents the centimeter error tolerance for RTK surveys
- It defines the cutoff elevation angle for satellite masking
Correct answer: It is the maximum acceptable geometric dilution before positional accuracy becomes unreliable for most GIS applications
A PDOP of 6 is widely accepted as the upper threshold for acceptable GPS survey conditions — values above 6 indicate poor satellite geometry and degraded positional accuracy that may be unsuitable for precise mapping.
Question 6: Which technique allows a GIS technician to collect GPS data along a linear feature such as a trail or road centerline?
- Waypoint averaging
- Track logging (Correct answer)
- Static occupation survey
- Network RTK
Correct answer: Track logging
Track logging continuously records GPS positions at set time or distance intervals as the receiver moves, creating a series of points that define a linear or areal feature's path.
GISP GPS and Field Data Collection 2
What is the primary purpose of a control point survey in establishing a geodetic network for GIS mapping projects?