Geospatial Data, GNSS and Digital Mapping
GNSS and digital mapping
GNSS positioning uses satellite ranges to determine position in a global reference frame. The exam tests code versus carrier-phase positioning, RTK/NRTK corrections, datum transformations, geoid use and digital map production. A fixed solution is not automatically a correct coordinate; the base station, datum and transformation must be controlled.
Core rules
- Use at least five satellites and good geometry; record fix status and PDOP.
- For RTK, know the base coordinate and correction link; check against a known point.
- Transform WGS84/ITRF positions to the local datum and grid, and apply geoid separation for heights.
- In GIS and digital mapping, record source, date, datum, accuracy and metadata.
- Use independent checks for cadastral or engineering coordinates.
Worked example
An RTK rover reports a fixed position in WGS84. The project requires JAD2001 grid coordinates. You transform the latitude and longitude using the published datum transformation and projection, then compare the result with a nearby control point. If the check is out by metres, you suspect the base coordinate or datum, not the rover. The reasoning is that RTK fixes the relative position well, but the absolute frame depends on the base and transformation.
Traps
Using autonomous GNSS for boundary work; ignoring base station coordinates; mixing ITRF epochs; treating ellipsoidal height as reduced level; and delivering GIS layers without metadata.