Geodesy, Coordinate Systems, and Control Surveys
What this domain tests
Geodesy questions test whether you can move between ground, ellipsoid, grid, and height systems without mixing datums or units.
Core rules
- NAD83(2011) is the horizontal reference for most Indiana work; NAVD88 is the common orthometric height datum. GNSS provides ellipsoid heights, so a geoid model is required for orthometric heights.
- Indiana State Plane has East and West zones. Grid distance differs from ground distance by the combined factor: grid ≈ ground × combined factor, where the combined factor combines projection scale and elevation factor.
- Grid azimuth differs from geodetic azimuth by convergence. Indiana also lies in UTM Zone 16, whose central meridian is 87° W; do not confuse UTM and State Plane coordinates.
- Control surveys should tie to published, verified control and state the datum, epoch, units, and adjustment.
Worked example
A ground distance is 1,000.000 ft and the combined factor is 0.99994. Grid distance = 1,000.000 × 0.99994 = 999.940 ft. Over one mile (5,280 ft), the grid-to-ground difference is about 5,280 × 0.00006 = 0.317 ft. To go from grid to ground, divide by the combined factor: 999.940 ÷ 0.99994 ≈ 1,000.000 ft.
Traps
Using surface distances directly in State Plane coordinates; mixing meters and U.S. survey feet; applying an orthometric height as an ellipsoid height; ignoring datum epoch; and treating convergence as zero everywhere.