Earth Gravitational Model

The Earth Gravitational Models (EGM) are a series of geopotential models of the Earth published by the National Geospatial-Intelligence Agency (NGA). They are used as the geoid reference in the World Geodetic System.

The NGA provides the models in two formats: as the series of numerical coefficients to the spherical harmonics which define the model, or a dataset giving the geoid height at each coordinate at a given resolution.

Three model versions have been published: EGM84 with n=m=180, EGM96 with n=m=360, and EGM2008 with n=m=2160. n and m are the degree and orders of harmonic coefficients; the higher they are, the more parameters the models have, and the more precise they are. EGM2008 also contains expansions to n=2190. Developmental versions of the EGM are referred to as Preliminary Gravitational Models (PGMs).

Each version of EGM has its own EPSG code as a vertical datum.