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Mexico Geoid Heights (MEXICO97)
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This 2' geoid height grid for Mexico, and North-Central America, is the MEXICO97 geoid
model. The computation used about one million terrestrial and marine gravity measurements
held in the NGS database as of March 1997. These gravity measurements were augmented
by data contributions from NGA (former National Imagery and Mapping Agency), and satellite
altimeter-derived gravity anomalies computed by Sandwell and Smith (1997). Large data
gaps south of 20 degrees North latitude were filled with 15'x15' gravity values derived
from the EGM96 global geopotential model. This helped control interpolation errors
across the data gaps during the gridding of terrain corrected Bouguer anomalies. After
gridding of the Bouguer anomalies, the Bouguer plate was restored, a degree 360 model
of gravity anomalies (from EGM96) was removed, and the residual free-air anomalies
in the data gaps (south of 20 degrees North Latitude) were zeroed. (This zeroing was
found to be the best way currently available to yield a reasonable geoid in the data
gaps). The residual Faye anomalies were converted to residual co-geoid undulations
through a 1-D FFT formulation of Stokes' integral, and finally the EGM96 undulation
model was restored, and the indirect effect applied. This means that in the data gaps,
long wavelength information is provided by EGM96, short wavelength information is
provided by the 2'x2' DTED (during the application of the indirect effect), but medium
wavelength information (usually provided by gravity measurements) is missing altogether.
Although the exact accuracy of the geoid in the data gaps is difficult to ascertain,
the quoted accuracy for EGM96 (which is the primary source of geoid information in
the data gaps) is below 50 cm in these areas. 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, centered at the origin of
the International Terrestrial Reference Frame 1994 (ITRF94(1996.0)). Additional information
is available at: http://www.ngs.noaa.gov/GEOID/MEXICO97/ We are particularly grateful
to NGA (former National Imagery and Mapping Agency) for their assistance and their
data contributions.
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