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Online geoid calculations using the
GeoidEval utility

GeoidEval
computes the height of the geoid above the WGS84 ellipsoid
using interpolation in a grid of values for the earth
gravity models,
EGM84, or
EGM96,
EGM2008.
The RMS error in the interpolated height is about 1 mm.
Give the position in terms of latitude and longitude, for example
(these all refer to the position of Timbuktu):

16.776 -3.009
16d47' -3d1'
W3°0'34" N16°46'33"
3:0:34W 16:46:33N

The coordinates can also be given in UTM, UPS, or MGRS coordinates (see
the documentation on the
GeoConvert utility).
The height of the geoid above the ellipsoid, *N*, is
sometimes called the geoid undulation. It can be used to convert
a height above the ellipsoid, *h*, to the corresponding
height above the geoid (the orthometric height, roughly the height
above mean sea level), *H*, using the relations

*h* = *N* + *H*;
*H* = −*N* + *h*.

GeoidEval,
which is a simple wrapper of the
GeographicLib::Geoid class,
is one of the utilities provided
with
GeographicLib.
This web interface illustrates a subset of its capabilities. If
you wish to use GeoidEval directly,
download
and compile GeographicLib. A description of the methods is given
here.

Charles Karney
<charles@karney.com>
(2014-11-06)
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