The gravitational field of the earth is non-uniform. The geoid is the shape the ocean surface would take if only gravity and the rotation of the Earth were considered. The geoid is the surface that defines zero elevation. The geoid height is the difference between an ideal reference ellipsoid and the geoid.
Variations in the geoid height are related to the density of the earth, so they can help us to better understand its internal structure. It is also used to measure surface elevations to high accuracy which is useful in applications such as GPS and surveying.
The official 2008 Earth Gravitational Model EGM2008 was developed and released to the public by the National Geospatial-Intelligence Agency (NGA). This gravitational model is complete to spherical harmonic degree and order 2159.
Our EGM2008 API provides on-demand access to the EGM2008 model, as implemented by the open-source GeographicLib Gravity library.
Geoid undulation values calculated using the API agree with corresponding values generated by the official EGM2008 harmonic synthesis program to within 1 mm.
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Try the API here.