The accleration due to gravity is given bywhereis the Gravitational constant, is the mass of the Earth andis the radius of the earth. The formula above assumes a spherically uniform distribution of mass, this this assumption is only approximately true. The Earth is not a sphere, being flattened at the poles, nor is the density of the Earth uniform. There are local mass concentrations – such as mountain ranges - which increase the acceleration due to gravity, and local deficits of mass. In the middle of the oceans the acceleration due to gravity is much weaker because water has only a fraction of the density of the continental crust which makes up the landmasses. The diagram below illustrates the variation in the acceleration due to gravity over the Earth's surface.

A milligal is a convenient unit for describing variations in gravity over the surface of the Earth. 1 milligal (or mGal) =which can be compared to the total gravity on the Earth's surface of approximatelyA milligal is about 1 millionth of the standard acceleration on the Earth's surface.