Assuming a spheroidal model for the Earth, acceleration due to gravity varies with latitude. A number of theoretical gravity formulas based on different spheroid models have been published. In this case, we will use the IAG 1980 formula based on the WGS84 spheroid model. The theoretical value gl is subtracted from the absolute gravity in calculating the free air anomaly (see below) to remove the gravity component contributed by the (assumed uniform) Earth.