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.
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).
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.