Undulation of the geoid

Somewhat old-fashioned name for the separation between the geoid and the reference ellipsoid of the Earth.

Because geophysical heights "above sea level" refer to the geoid, but GPS positioning provides in a direct geometric fashion heights above the GRS80 reference ellipsoid, it has become a matter of great importance to determine accurate representations, or maps, of geoid undulations

over the territories of nations, to allow the inexpensive GPS technology to be harnessed for the precise determination of heights.