Earth Gravity Model 1996 (EGM96)

The most common global geopotential model is the Earth Gravity Model 1996 (EGM96).

EGM96 was published jointly by the United States National Aeronautical and Space Agency (NASA) and the National Imagery and Mapping Agency (NIMA) in 1998. Technical details of the EGM96 model are available on the NASA website - NASA GSFC and NIMA Joint Geopotential Model.

The estimated accuracy of the EGM96 model is 1-2 metres at its maximum resolution of 56 kilometres. This means that gravity field changes that are smaller than this resolution will not be depicted by it. Similarly the 1-2 metre accuracy level means that an error of this size may be introduced into heights that use the EGM96 in their calculation. In comparison the NZGeoid05 has an estimated accuracy of 8 centimetres at a resolution of 3.7 kilometres.

EGM96 is available from the NASA website. It is provided in terms of spherical harmonic coefficients which generally need to be converted into a grid of geoid undulations before they can be used.