Undulation of the geoid

Undulation of the geoid is a somewhat old-fashioned name for the separation $N$ between the geoid and the reference ellipsoid of the Earth.

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

$N\; =\; h\; -H,,$

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

References

* [http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003C.HTM "CHAPTER V PHYSICAL GEODESY"] , from the [http://www.ngs.noaa.gov/ National Geodetic Survey web site]