The Geoid and Geoid Models Review
The geoid is
that equipotential surface which would coincide with the mean ocean
surface of the Earth, if the oceans and atmosphere were in equilibrium, at rest
relative to the rotating Earth, and extended through the continents (such as
with very narrow canals). A geoid is the "mathematical figure of the
Earth", a smooth but highly irregular surface that corresponds not to the
actual surface of the Earth's crust, but to a surface which can only be known
through extensive gravitational measurements and calculations.
1=Ocean;
2=Reference ellipsoid; 3=Local plumb line; 4=Continent; 5=Geoid
The
gravity field of the earth is neither perfect nor uniform. A flattened
ellipsoid is typically used as the idealized earth, but even if the earth were
perfectly spherical, the strength of gravity would not be the same everywhere,
because density (and therefore mass) varies throughout the planet. This is due
to magma distributions, mountain ranges, deep sea trenches, and so on.
If
that perfect sphere were then covered in water, the water would not be the same
height everywhere. Instead, the water level would be higher or lower depending
on the particular strength of gravity in that location.
Source: NGS, Wickipedia
NGS Geoid09
GEOID09
is a refined hybrid model of the geoid in the United States and other
territories, which supersedes the previous models GEOID06, GEOID03,
GEOID99, GEOID96, GEOID93, and GEOID90. This model is intended for converting
between the NAD83 ellipsoid reference frame and vertical datum NAVD88, GUVD04
(Guam), ASVD02 (American Samoa), NMVD03(Northern
Marianas), PRVD02 (Puerto Rico) and
VIVD09 (Virgin Islands).
http://www.ngs.noaa.gov/GEOID/GEOID09/
NGA EGM2008
The
official Earth Gravitational Model EGM2008 has been publicly released by the
U.S. National Geospatial-Intelligence Agency (NGA) EGM Development Team. This
gravitational model is complete to spherical harmonic degree. Full access to
the model's coefficients and other descriptive files with additional details
about EGM2008 are provided at the NGA website.
Those
wishing to use EGM2008 to compute geoid undulation values with respect to
WGS 84, may do so using the self-contained suite of coefficient files,
FORTRAN software, and pre-computed geoid grids provided on the NGA website.
