The gravitational field of the earth is not 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.
The gravity anomaly is the difference between the acceleration due to gravity on the Earth's surface and the value calculated assuming the reference ellipsoid.
The official Earth Gravitational Model EGM2008 was developed and released to the public by the National Geospatial-Intelligence Agency (NGA).
Our EGM2008 API provides on-demand access to the EGM2008 model, as implemented by the open-source GeographicLib Gravity library.