Undulation of the geoid
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Somewhat old-fashioned name for the separation <math>N<math> between the geoid and the reference ellipsoid of the Earth.
Because geophysical heights <math>H<math> "above sea level" refer to the geoid, but GPS positioning provides in a direct geometric fashion heights <math>h<math> above the GRS80 reference ellipsoid, it has become a matter of great importance to determine accurate representations, or maps, of geoid undulations
<math> N = h -H,\, <math>
over the territories of nations, to allow the inexpensive GPS technology to be harnessed for the precise determination of heights.