Geodesy and Positioning
What the domain tests
Geodesy and Positioning tests the reference surface, datum, projection, and GNSS model behind every coordinate. Expect questions on geoid-ellipsoid separation, datum transformation, map projection scale, and positioning quality.
Core rules
- The geoid is an equipotential surface approximating mean sea level; the ellipsoid is the mathematical reference. Orthometric height is above the geoid; ellipsoid height is above the ellipsoid; the difference is the geoid separation (N).
- A geodetic datum fixes the ellipsoid to the earth; coordinates are meaningless without datum, epoch, and projection.
- Projection scale factor converts ellipsoid distance to grid distance; elevation factor converts ground distance to ellipsoid distance. Combined factor = scale factor × elevation factor.
- GNSS gives ellipsoid heights and requires a geoid model for orthometric heights. RTK/CORS positions need independent checks and correct datum transformation.
- Accuracy depends on satellite geometry, observation time, baseline length, and correction integrity-not just “fix” status.
Worked example
Ground distance 1000.000 m at ellipsoid height 200 m. Mean Earth radius 6,371,000 m. Elevation factor = R/(R+h) = 6,371,000/6,371,200 = 0.9999686. If projection scale factor is 0.9996, combined factor = 0.9995686. Grid distance = 1000.000 × 0.9995686 = 999.569 m. The reasoning: ground distance is first reduced to the ellipsoid, then to the grid; applying only the projection factor ignores height.
Traps
Calling ellipsoid height “elevation”; applying scale factor the wrong way; using WGS84 coordinates directly as cadastral coordinates; and accepting a GNSS fix without redundancy.