Geodesy and Coordinate Systems
What the exam tests
Geodesy items test datums, ellipsoid versus geoid, map projections, grid versus ground distances, convergence, and elevation factors. Expect numerical conversions.
Surfaces and datums
- NAD83(2011) is a geometric datum referenced to an ellipsoid; NAVD 88 is an orthometric height datum referenced to the geoid.
- Ellipsoid height h, orthometric height H, and geoid separation N relate as h = H + N.
- State Plane zones use Lambert Conformal Conic or Transverse Mercator; UTM uses Transverse Mercator zones with a central meridian scale factor of 0.9996.
Grid versus ground
A combined factor is the product of the elevation factor and the grid scale factor. Ground distance equals grid distance divided by the combined factor. Convergence is the angle between true north and grid north; geodetic azimuth differs from grid azimuth by convergence.
Worked example
A grid distance is 1,000.000 ft and the combined factor is 0.99987. Ground distance = 1,000.000 / 0.99987 = 1,000.130 ft. The grid distance is shorter because the projection and elevation reduced the ground distance. Multiplying by 0.99987 would give 999.870 ft, which is the wrong direction.
Traps
- Applying the combined factor in the wrong direction.
- Using only the elevation factor and ignoring projection scale.
- Mixing ellipsoid heights with NAVD 88 elevations.
- Ignoring convergence when reporting geodetic azimuth.
- Assuming a GIS reprojection changed the underlying coordinates.