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.

 

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.

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.

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.