Geodesy, Control, and Mapping
Geodesy, Control, and Mapping
The PS exam tests geodetic reference systems, state plane coordinates, control networks, and map products. Expect datum transformations, scale factors, geoid relationships, and topographic mapping.
Datums and heights
NAD83 is the horizontal datum; NAVD88 is the vertical datum. Ellipsoidal height h is measured from the ellipsoid; orthometric height H is measured from the geoid. The geoid separation N satisfies H = h − N. A geoid model such as GEOID18 converts GNSS ellipsoid heights to NAVD88 orthometric heights.
State plane coordinates
Grid distance differs from ground distance because of projection scale and elevation. Grid scale factor (GSF) accounts for projection distortion; elevation factor (EF) accounts for height above the ellipsoid. Combined factor = GSF × EF. Ground distance = grid distance ÷ combined factor; grid distance = ground distance × combined factor. State plane zones also have convergence angles between grid north and true north.
Control and mapping
Horizontal control is hierarchical: NSRS, CORS, then local ties. Topographic maps show contours; contour interval is the vertical distance between adjacent contours. GIS data must be referenced to a datum and projection.
Worked example
A state plane grid distance is 10,000.000 ft and the combined factor is 0.99995. Ground distance = 10,000.000 ÷ 0.99995 = 10,000.500 ft. Reason: the grid distance is slightly shorter than ground because the combined factor is less than 1; dividing by a factor below 1 increases the distance.
Traps
- Using ellipsoid height as orthometric height.
- Multiplying by the combined factor when converting grid to ground.
- Ignoring convergence when reporting bearings.
- Confusing contour interval with index contour.
- Assuming a datum transformation is zero-parameter.