- Undulation of the geoid
Undulation of the geoid is a somewhat old-fashioned name for the separation N between the
geoid and thereference 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 theGRS80 reference ellipsoid , it has become a matter of great importance to determine accurate representations, or maps, of geoid undulationsN = 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]
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