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DC Land Surveyor Exam study materials
DC Land Surveyor Exam study resources

DC Land Surveyor Exam: Geodesy and Coordinate Systems

DC Land Surveyor Exam · Work through your exam, one topic at a time.

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Geodesy and Coordinate Systems

Geodesy questions test whether you can move between the ellipsoid, the grid, and the ground without losing a tenth of a foot. DC work typically involves NAD83, NAVD88, Maryland State Plane coordinates, and GNSS-derived ellipsoid heights.

Reference surfaces and frames

  • NAD83 uses the GRS80 ellipsoid. Orthometric heights are NAVD88; GNSS gives ellipsoid heights. The relation is H = h - N, where N comes from a geoid model.
  • NGS is modernizing the National Spatial Reference System, replacing NAD83 and NAVD88 with NATRF2022 and NAPGD2022. Existing DC surveys remain on NAD83(2011) and NAVD88.
  • The District lies within the Maryland North zone of the State Plane Coordinate System, a Lambert Conformal Conic projection commonly used in US survey feet. Local ground coordinate systems are also used for construction.

Grid versus ground

  • Grid distance = ground distance × combined factor, where the combined factor is the projection scale factor times the elevation factor, R/(R+h).
  • The elevation factor is less than 1 for points above the ellipsoid; the scale factor is also usually slightly less than 1 in this zone, so grid distances are shorter than ground distances.
  • Convergence of meridians means grid azimuth differs from geodetic azimuth; the difference grows away from the central meridian.
  • OPUS, CORS stations, and NGS datasheets are the normal sources of control and datum information.

Worked example

A ground distance of 1,250.000 ft is measured at an ellipsoid height of 250 ft, with a scale factor of 0.99993500. Take R = 20,906,000 ft. Elevation factor = 20,906,000 / 20,906,250 = 0.99998804. Combined factor = 0.99993500 × 0.99998804 = 0.99992304. Grid distance = 1,250.000 × 0.99992304 = 1,249.904 ft, a difference of 0.096 ft.

Traps that cost marks

  • Mixing meters and US survey feet, or international feet and US survey feet.
  • Using an ellipsoid height as an orthometric elevation.
  • Multiplying by the combined factor when converting grid to ground instead of dividing.
  • Applying a scale factor to a short boundary line and claiming a precision the control does not support.
  • Ignoring convergence when comparing GNSS azimuths with total-station bearings.