geoid

The equipotential gravimetric surface (a surface of equal gravity potential) that closely approximates mean sea level. An equipotential surface is normal to the gravity vector at every point. The geoid surface is not a uniform mathematical shape, but is an irregular figure with an overall shape similar to an ellipsoid.

Generally, the elevations of points are measured with reference to the geoid. However, points fixed by GNSS methods have heights established in the global reference datum (a mathematical figure). The relationship between the global reference datum and the geoid must be determined by observation, as there is no single mathematical definition that can describe the relationship.

You must use conventional survey methods to observe the elevation above the geoid, then compare the results with the height above the global reference datum at the same point. By gathering a large number of observations of the separation between the geoid and the global reference datum (geoidal separation), grid files of the separation values can be established. These grid files allow the interpolation of the geoidal separation at intermediate positions. Files containing these grids of geoidal separations are referred to as geoid models. Given a global position that falls within the extents of a geoid model, the model can return the interpolated geoidal separation at a specified position.