In order to derive geologically meaningful anomalies, the effects of known non-geological factors must be removed. As shown in Figure 1 in highly exaggerated form, the Earth is an ellipsoid not a sphere (the actual flattening of the Earth is about 1 part in 300). Thus at sea level, the gravitational attraction of the Earth is diminished at the equator because this is the place where one is farthest from the center of mass of the Earth. Gravity at the equator is further diminished because the rotation of the Earth creates a centrifugal acceleration (C) that opposes gravity. This acceleration is greatest at the equator because the distance to the rotational axis (R) is the greatest (C = RW², whereW is the angular velocity of the Earth’s rotation). These effects are partly offset by the fact that the equatorial bulge effectively produces extra mass at the equator. The International Gravity Formula that takes these effects into account is of the form:

G_t = G_e (1 + f₁sin q² + f₂sin q⁴), where

• G_e is the average value of gravity at the equator ~978 Gal
• qis the latitude
• f₁, f₂depend on the flattening of the Earth and its gravity field, and the centrifugal force

The effects of elevation also have to be considered before a geologically meaningful anomaly can be calculated. The “Free effect” is the decrease of gravity with elevation due to the increased distance from the Earth’s center of mass. This effect is the vertical gradient of the gravity field whose value is about 0.3 mGal/m and is negative. However, any location above the datum has extra mass beneath it. The effect of this mass is calculated via a volume integral, where an average density of the topography must be estimated (the value used in regional studies is 2670 kg/m³). This effect (the Bouguer correction) is opposite in sign to the Free Air effect and is about 1/3 of its value. Thus, the total effect of elevation is to decrease gravity by ~ 0.2 mGal/m.

When these effects of elevation are considered, the results is a geological meaningful value for the Bouguer anomaly. In order to fully account for the effects of topography, a terrain correction must be calculated. If this is done, a complete Bouguer anomaly value results.