1. A correction to gravity data because of (a) variation in centrifugal force resulting from the Earth’s rotation, as the distance to the Earth’s axis varies with latitude , and (b) variation of the Earth’s radius because of polar flattening. The Geodetic Reference System 1967 (GRS67) gives
The latitude correction amounts to
2. A gyrocompass correction for the rotation of the horizontal north vector as a function of latitude. (The horizontal north vector is tangent to the Earth and hence the rotation is the result of Earth curvature.)
3. The north-south correction to magnetic data to remove the Earth’s normal field.