The National Imagery and Mapping Agency (NIMA), has been of immense help in this endeavor. NIMA has provided a major portion of the NGS land gravity data set. NIMA has also been instrumental in the creation of the various 30" and 3" elevation grids in existence. And, NIMA was a partner in the joint project to compute the new global geopotential model, EGM96. Although the work of NIMA generally precludes public recognition, their cooperation is gratefully acknowledged.
The Goddard Space Flight Center (GSFC) and the National Imagery and Mapping Agency (NIMA) were engaged in a joint project to compute an improved global spherical harmonic model of the Earth's geopotential. This model incorporated the latest satellite tracking data, as well as altimeter data from TOPEX/Poseidon, ERS-1, and the Geosat Geodetic Mission. EGM96 also incorporated new surface and marine gravity data covering the globe, including the former Soviet Union. EGM96 is a global geopotential model expressed as spherical harmonic coefficients complete to degree and order 360. Therefore, the shortest wavelength this model can exhibit is one degree, and its resolution is one-half degree (about 50 km). Although this model does not reproduce geoid structure at very fine resolution, it is global. We thank the many members of the project team for making this model available.
The United States Geological Survey (USGS), has for years been producing high resolution digital elevation models for the United States. Using the highest resolution (10 and 30 meter) DEMs from USGS, the National Geodetic Survey created a new 1 arcsecond DEM for the Northwest United States, NGSDEM99. This new DEM significantly improved our knowledge of the gravimetric geoid in the Northwest. We thank USGS for making their raw DEM data available to us.