ISOSTATIC GRAVITY, 4 KM

The isostatic residual gravity grid was made using an Airy- Heiskanen model of local compensation with a depth of 30.0 km to the bottom of the root for sites with sea level elevations and a density contrast of 0.35 g/cm*3 across the bottom of the root. The isostatic grid was derived by the U.S. Geological Survey from the gravity data set used to prepare the Gravity Anomaly Map of the United States published by the Society of Exploration Geophysicists and from the 5-minute North American topographic and SYNBAPS bathymetric data sets obtained from the NOAA data center.

In the descriptions that follow, the abbreviations “res”, “reg” and “top” in the file name indicate Residual, Regional and Topographic grids. Note that the parameters for each grid are slightly different, but that they overlay the same geographical area --the conterminous United States.

NOTE: The binary files isores.bin, isoreg.bin, and isotop.bin were converted from the ASCII files on the Geophysics of North America CD-ROM by Ted Habermann. The data values were converted to 4 byte binary integers with two digits to the right of the decimal point. These integers must, therefore, by divided by 100 to recover the original data. The file isostat.bin is a binary file copied from the Geophysics of North America CD-ROM.
 

HEADER RECORD CONTENTS (240 BYTES) - Isostatic Gravity Residual:

U.S. Isostatic Residual
d=30km, rho=0.35g/cc gsubtr ncol= 1300 nrow= 772 nz= 1
xo=-.25740000E+04 dx=0.40000000E+01%
yo=0.25390000E+04 dy=0.40000000E+01

% indicates a continuation of this logical record on the next text line OF THIS DOCUMENT ONLY!
NOTE: that this % IS NOT actually recorded in the header record section of this data file.

HEADER RECORD FORMAT:

1. 80 alphanumeric characters of identification. The identification is: U.S. Isostatic Residual d=30km, rho=0.35g/cc gsubtr recorded in a A64, 16x format.

2. The number of columns of the grid. This value is 1300 and is recorded in I8 format; six blank characters precede this field.

3. The number of rows of the grid. This value is 772 and is recorded in I8 format; eight blank characters precede this field.

4. The number of floating point values stored for each Grid point. Note that for a real grid, nz = 1; for a complex grid, nz = 2. The value is 1 for this data set and is recorded in I8 format. Six blanks precede this field. Thirty-six blanks follow this field.

5. The x coordinate of the first column. The value is -.25740000E+04 and is the distance in kilometers west of the central meridian longitude of 96 degrees West. It is recorded in E14.8 format. Five blank characters precede this field.

6. The distance in kilometers between the columns of the grid. This value is 0.40000000E+01 and is recorded in E14.8 format. The field is preceded by five blanks.

7. The y coordinate of the first row. The value is 0.25390000E+04 and is the distance in kilometers north of the equator. It is recorded in E14.8 format, where five characters must be skipped before reading this field.

8. The distance in kilometers between the rows of the grid. The value is 0.40000000E+01 and is recorded in E14.8 format. Thus the first 240 bytes of this data file are header information which can be read with the following Fortran format statement:

100 FORMAT

( A64, 16X, + 6X, I8, 8X, I8, 6X, I8, 36X, + 4(5X, E14.8),4X)

The data are binary and are in 4 byte integers. The integer value must be divided by 100 to get the actual value with two digits to the right of the decimal point. A histogram showing the distribution of these data is in the file \lists\isores.lst

Original File = 4,045,520 bytes
1300 columns X 772 rows = 1,003,600
data 4 bytes / data point = 4,014,400 bytes
+ 240 byte header = 4,014,640 bytes

HEADER RECORD CONTENTS (240 BYTES) - Isostatic Regional:

U.S. Isostatic Regional

(d=31.7, rho=.385) gsubtr ncol= 650 nrow= 386 nz= 1
xo=-.25740000E+04 dx=0.80000000E+01%
yo=0.25390000E+04 dy=0.80000000E+01

% indicates a continuation of this logical record on the next text line OF THIS DOCUMENT ONLY!
NOTE: that this % IS NOT actually recorded in the header record section of this data file.

The definitions of these fields are the same as those for the file isostat.res.

Data Records:

Original File = 4,045,520 bytes

650 columns X 386 rows = 250,900 data
4 bytes / data point = 1,003,600 bytes
+ 240 byte header = 1,003,840 bytes

HEADER RECORD CONTENTS (240 BYTES) - 5 minute topo projected:

U.S. 5 minute topo projected to 8km with albers
(96,0) gsuperc ncol= 740 nrow= 476 nz= 1
xo=-.29340000E+04 dx=0.80000000E+01%
yo=0.21790000E+04 dy=0.80000000E+01

% indicates a continuation of this logical record on the next text line OF THIS DOCUMENT ONLY!
NOTE: that this % IS NOT actually recorded in the header record section of this data file.

The definitions of these fields are the same as those for the file isostat.res.

Data Records:

Original File = 5,674,160 bytes

740 columns X 476 rows = 352,240 data
4 bytes / data point = 1,408,960 bytes
+ 240 byte header = 1,409,200 bytes

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