Directory Identifier: GRAVCD-isostat
Directory Title: 2.5-min Isostatic Gravity Grid for the United States
Principal Investigator: Dr. Richard Hansen

Contributing Organization: Cooperative Institute for Geoscience Data Management and Applications (CIGMA)

Summary: The 2.5-min isostatic gravity data set was produced by regridding the 4-km residual isostatic gravity grid of the U.S. The isostatic residual gravity grid was developed by the U.S. Geological Survey using an Airy Heiskanen model. Local compensation was computed with a depth of 30.0 km to the root for sites with sea level elevations and a density contrast of 0.35 g/cm3 across the bottom of the root.

The isostatic grid was derived from the gravity data set used to prepare the Gravity Anomaly Map of the United States (1982) published by the Society of Exploration Geophysicists, and from the 5-minute North American topographic and SYNBAPS bathymetric data sets obtained from the National Geophysical Data Center. Contour and colored maps of the isostatic residual gravity data have been published.

Additional information is available.

There are four United States isostatic gravity data files on the CD-ROM:

ISOSTAT - Residual isostatic gravity in a 2.5-minute latitude/longitude grid with values in mgals. This file was developed by Richard Hansen of the Cooperative Institute for Geoscience Data Management and Applications (CIGMA), using the 4- km isostatic grid described below.

ISORES - Residual isostatic gravity in a 4-km grid using an Albers projection (central meridian = 96-degrees; base latitude = 0-degrees) with values in mgals. These data were computed from the following two 8-km grids which were also in an identical Albers projection. (Note: Isostatic Residual = Bouguer Isostatic Regional).

ISOREG - Sea level isostatic regional data in a 8-km grid. These data are in units of mgals.

ISOTOP - Topographic data for the conterminous U. S. in a 8-km grid. These data are in units of meters. isostat.bin

2.5 minute grid of Isostatic gravity anomalies for the U.S.A. (from GNA).

GRID LIMITS:
longitude : 128W to 63.95833W
Latitude : 23N to 49.54167N
637 Rows X 1537 Columns = 979069 data
2 bytes / data point = 1,958,138 bytes
The data are stored row wise starting in the upper left corner.

Isostatic Gravity References

Godson, R.H., and Scheibe, D.M., 1982, Description of magnetic tape containing conterminous U.S. gravity data in gridded format: U.S. Dept. of Commerce, National Technical Information Service, PB82-254798 (magnetic tape with description), 5 p.

Jachens, R.C., Simpson, R.W., Saltus, R.W., and Blakely, R.J., 1985, Isostatic residual gravity anomaly map of the United States (exclusive of Alaska and Hawaii): National Oceanic and Atmospheric Administration, National Geophysical Data Center map on clear film, scale 1:2,500,000.

Simpson, R.W., Jachens, R.C., Blakely, R.J., 1983, AIRYROOT: A Fortran program for calculating the gravitational attraction of an Airy isostatic root out to 166.7 km: U.S. Geological Survey Open-File Report 83:883, 66 p.

Simpson, R.W., Saltus, R.W., Jachens, R.C., and Godson, R.H., 1983, A description of colored isostatic gravity maps and a topographic map of the conterminous United States available as 35- mm slides: U.S. Geological Survey Open File Report 83-884, 16 p.

Simpson, R.W., Jachens, R.C., Saltus, R.W., and Blakely, R.J., 1985, Isostatic residual gravity maps, topographic, and first- vertical-derivative gravity maps of the conterminous United States: U.S. Geological Survey Geophysical Investigations Map GP-975, scale 1:7,500,000.

Society of Exploration Geophysicists, 1982, Gravity anomaly map of the United States (exclusive of Alaska and Hawaii): Society of Exploration Geophysicists, scale 1:2,500,000.

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