Geodetic and Control Surveying
What this domain tests
Datum literacy and control design: moving between ellipsoid, geoid, grid, and ground, and knowing which control and accuracy standard a project requires.
Datums and heights
- Horizontal: NAD83 (and its 2011 realization) on the GRS80 ellipsoid; the modernized NATRF2022 replacement is coming. Vertical: NAVD88, with NAPGD2022 replacing it.
- Ellipsoid height h, orthometric height H, geoid undulation N: H = h − N. GNSS gives h; leveling gives H.
Grid versus ground
- North Dakota uses the State Plane Coordinate System, 1983 datum, Lambert conformal conic, with a North zone and a South zone; confirm whether your project is published in meters or U.S. survey feet.
- Grid distance = ground distance × combined factor, where combined factor = grid scale factor × elevation factor.
- Elevation factor ≈ R / (R + h), with R ≈ 20,906,000 ft.
- Mapping angle (convergence) rotates grid azimuth to geodetic azimuth.
Control practice
Design networks with redundancy and closed loops; check level loops by misclosure against the order's allowable; check GNSS by comparing repeated baselines and known vectors. Positional tolerance is normally stated at 95% confidence, not as an average error.
Worked example
H = 2,000 ft, grid scale factor = 0.9999300. Elevation factor = 20,906,000 / 20,908,000 = 0.9999043. Combined factor = 0.9999300 × 0.9999043 = 0.9998344. A grid distance of 1,000.00 ft corresponds to 1,000.00 / 0.9998344 = 1,000.17 ft on the ground. To go ground-to-grid you multiply; to go grid-to-ground you divide.
Traps that cost marks
- Applying a scale factor in the wrong direction, or applying it to an area without squaring it.
- Substituting ellipsoid height for orthometric height (or the reverse) in a elevation-factor calculation.
- Mixing meters and feet between zones or between software and the record.
- Ignoring epoch and tectonic motion when connecting to NAD83(2011) coordinates.