This 2? geoid height grid for the conterminous United States is the G96SSS model.
The computation used about 1.8 million terrestrial and marine gravity data held in
the National Geodetic Survey gravity data base in July 1996. These data were augmented
by gravity data contributions from NGA (former National Imagery and Mapping Agency
(former Defence Mapping Agency)). By means of a Fast Fourier Transform (FFT) technique,
high frequency corrections were made to an underlying EGM96 geopotential model through
a remove, compute, and restore process. The gravity values are based on the International
Gravity Standardization Net 1971 (IGSN71). The geoid heights are referred to the Geodetic
Reference System 1980 (GRS80) ellipsoid. Unlike GEOID96, the G96SSS grid does not
incorporate GPS on leveled benchmarks. The G96SSS model is a gravimetric geoid in
a geocentric, ITRF94(1996.0) reference frame. It is necessary to subtract 12.0 cm
from the G96SSS values to obtain the geoid undulation between the best-fit global
geopotential surface and the GRS80 ellipsoid (both expressed in a tide free system).
Additional information is available at: http://www.ngs.noaa.gov We are particularly
grateful to NGA (former National Imagery and Mapping Agency) for their assistance
and their data contributions. |