EGM96: The <A HREF=""> NASA GSFC </A> and <A HREF=""> NIMA </A> Joint Geopotential Model


EGM96 General Description

EGM96 is a geopotential model of the Earth consisting of spherical harmonic coefficients complete to degree and order 360. It is a composite solution, consisting of: (1) a combination solution to degree and order 70; (2) a block diagonal solution from degree 71 to 359; and (3) the quadrature solution at degree 360. This model, just completed, is the result of a collaboration between the National Imagery and Mapping Agency, the NASA Goddard Space Flight Center, and the Ohio State University.

The joint project toook advantage of new surface gravity data from many different regions of the globe, including data newly released from the NIMA archives. Major terrestrial gravity acquistions by NIMA since 1990 include airborne gravity surveys over Greenland and parts of the Arctic and the Antarctic, surveyed by the Naval Research Lab (NRL), and cooperative gravity collection projects, several which were undertaken with the University of Leeds. These collection efforts have improved the data holdings over many of the world's land areas, including Africa, Canada, parts of South America and Africa, Southeast Asia, Eastern Europe, and the former Soviet Union. In addition, there have been major efforts so improve NIMA's existing 30' mean anomaly database through contributions over various countries in Asia.

NIMA also computed and made available 30'x30' mean altimeter derived gravity anomalies from the GEOSAT Geodetic Mission. EGM96 also included altimeter derived anomalies derived from ERS-1 by Kort and Matrikelstyrelsen (KMS), (National Survey and Cadastre, Denmark) over portions of the Arctic, and the Antarctic, as well as the altimeter derived anomalies of Schoene [1996] over the Weddell Sea. The statistics of the final merged set of surface gravity data used in EGM96 are summarized in the table below.

Statistics of the final EGM96 global 30' mean free-air
gravity anomaly merged file

Terr. 30 min
Altim. 30 min
Terr. 30 min
Terr 1 deg
Fill-in 30 min
of cells
86740 146042 1064 6500 18854
of area
30.7 66.1 0.1 0.8 2.3
RMS of
data (mgal)
35.2 25.6 56.7 49.1 28.0
RMS of data
std deviation
5.4 1.7 16.9 35.7 36.0

Other data that contributed to EGM96 are direct satellite altimetry from TOPEX/POSEIDON, ERS-1 and GEOSAT, and satellite tracking to over 20 satellites using DORIS , Tracking and Data Relay Satellite System (TDRSS), and TRANET. In addition, as part of this effort a new topographic data base was assembled from all available sources in order to reduce the raw surface gravity data, and prepare the 30' x 30' gravity anomalies. This database was also used to calculate the isostatic fill-in values in the roughly 3 percent of the globe where surface gravity data was not available. The geographical extent and availability of data is a major improvement over the surface gravity data available for OSU91A.

This new geopotential model will be used as a geodetic reference to update WGS-84. In addition, it will serve as a state of the art model for precise orbit, oceanographic, and geophysical studies.

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Last revised June 17,2005, 18:03 UTC.