2. Geodesy
Geodesy
This domain tests the Earth model, geoid, ellipsoid, geodetic coordinates and the relationship between physical and mathematical heights. Expect definitions, height relationships and the reason a datum is needed before coordinates.
Core rules
- The geoid is an equipotential surface approximating mean sea level; the ellipsoid is a mathematical reference surface.
- Geodetic latitude and longitude refer to the ellipsoid normal; astronomical coordinates refer to the gravity vertical.
- Orthometric height H, ellipsoidal height h and geoid separation N satisfy h = H + N.
- Deflection of the vertical is the angle between the ellipsoid normal and the gravity vertical.
- A geodetic datum fixes an ellipsoid to the Earth and defines the origin and orientation of coordinates.
Worked example
A point has orthometric height 120.0 m and geoid separation N = -95.0 m. Its ellipsoidal height is h = H + N = 120.0 + (-95.0) = 25.0 m. The negative N is normal in Sri Lanka and means the ellipsoid lies above the geoid there. Do not report 215.0 m by adding without sign.
Traps
Mixing ellipsoidal and orthometric heights; treating the geoid as a sphere; ignoring deflection when comparing astronomical and geodetic azimuths; applying a datum transformation to heights without a geoid model.