Contributing Organization: National Geophysical Data Center (NGDC)

Summary: The dgrav gridded data set was produced at NGDC by regridding the Decade of North American (DNAG) 6-km gravity grid of N. America. A grid cell dimension of 2.5 minutes of longitude and latitude was used in this regridding process. In order to facilitate the comparison of the various regridded data, a geodetic reference system was used as the most suitable common coordinate frame. The Brigg's minimum curvature algorithm was chosen for a gridding procedure since the majority of these data sets represent potential field estimates. We used an implementation of the Briggs algorithm written by Mike Webring of the U.S. Geological Survey. The Webring code allowed us to generate grids from randomly distributed data. In addition to the grid cell dimension of 2.5 minutes of longitude and latitude other gridding parameters were chosen so that flagged areas containing no data would correspond to the published DNAG maps. The 2.5 minute grid cell dimension led to a high degree of spatial distortion at very high latitudes. Therefore, we decided to truncate the latitude-longitude grids above 80 degrees North.

NOTE: Regridding the potential field data from kilometers on a map projection into latitude and longitude introduced some gradient distortions at all latitudes. We would recommend that anyone attempting to do quantitative work with the potential field grids use the original 6-km gravity grids. DNAG GRAVITY, 2.5 Minute: dgrav.doc This file is a binary file copied from the Geophysics of North America CD-ROM. It contains bouguer/free-air gravity anomalies for North America on a 2.5 minute grid File dgrav.bin - 2.5 minute grid of bouguer/free-air gravity anomalies for the U.S.A. (from GNA). GRID LIMITS: Longitude: 170E to 10.75W Latitude: 5.16667N to 81.1667N 1824 Rows X 4302 Columns = 7,846,848 data 2 bytes / data point = 55,693,696 bytes. The data are stored row wise starting in the upper left corner.

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