Description of CD-ROM Disk File
containing
Gridded Gravity Anomaly Values
of the
Gravity Anomaly Map of North America
Introduction
The grid values contained in this file were used to produce
the Gravity Anomaly Map of North America (The Committee for the Gravity
Anomaly Map of North America, 1987, scale 1:5,000,000.) Considerable caution
should be exercised when using the 6 km gridded data in regions of sparse
data coverage, for example, deep oceanic areas. The spatial distribution
of the original data from which the grid was generated is shown on sheet
5 of the published North American Gravity Map. The only exception to be
noted is that there are no gridded data values for the landmass of Mexico.
CD-ROM Disk File Format
The data are stored on the CD-ROM disk in an integer format
of milligals times ten. The file contains 2,137,850 (1430 x 1495) values.
Please NOTE that 2 extra bytes of the “blank” ASCII character (20 Hex)
are written at the upper most end of each column. Areas of no data have
been assigned values of -9999.
The file of the gridded data consists of two basic parts:
(1) a sequence of identification or header bytes followed by (2) a columnar
series of 1430 data values (in F5.1 Fortran Format) ending with two blank
bytes. A column is defined as a series of data positions that extend in
a south to north direction along a common west coordinate. The columns
increase in an easterly direction.
The data in this grid are in a Spherical Transverse Mercator
projection, the kilometer coordinates of which can be recovered from the
indices of a grid point. If IROW and JCOL are the row and column position
of a grid point then the abscissa, XKM and ordinate, YKM may be computed
in kilometers of the projection as
XKM = X0 + ( JCOL - 1 ) * DELTX
YKM = Y0 + ( IROW - 1 ) * DELTY
where X0, Y0, DELTX, and DELTY have the definitions given
in the next paragraph describing the header record.
A more complete description of this projection and procedures
for transforming these coordinates into latitude and longitude may be found
in the users guide for this CD-ROM.
Header Record
The header information sequence is 7152 bytes long. It
may be read with the following Fortran “READ”:
READ(10,100) ID,NCOL,NROW,XO,DELX,YO,DELY 100
FORMAT (A64, 2I5, 2(F6.0, F2.0), 7062X)
These fields are defined as follows:
- ID - 64 alphanumeric characters of identification. The identification is: 6 KM GRIDDED GRAVITY DATA OF NORTH AMERICA, recorded
in A64 format.
- NCOL - The number of columns of the grid. This value is 1495 and is recorded in I5 format.
- NROW - The number of rows of the grid. This value is 1430 and is recorded in I5 format.
- XO - The x coordinate of the first column. The value is -4480 and is the distance in kilometers west of the central meridian
longitude 100 deg. W. It is recorded in F6.0 format.
- DELX - The distance in kilometers between the columns of the grid. This value is recorded in F2.0 format.
- YO - The y coordinate of the first row. The value is 700. and is the distance in kilometers north of the equator.
It is recorded in F6.0 format.
- DELY - The distance in kilometers between the rows of the grid. The value is recorded in F2.0 format.
- 7062 blank characters to fill out the record.
Data Records
There are 1495 logical data records following the header
record.
The following diagram shows the relationship of the grid
elements.
1430 |
X |
X |
X |
X |
X |
: |
X |
X |
X |
X |
X |
2 |
X |
X |
X |
X |
X |
1 |
X |
X |
X |
X |
X |
|
1 |
2 |
3 |
.. |
1495 |
|
A |
|
|
|
Z |
Note: A = First column stored on tape.
Z = Last column stored on tape.
The FILE can be read in FORTRAN with the following statements:
C |
GRAVITY FILE MUST BE ASSIGNED TO FORTRAN LOGICAL UNIT
10 |
|
DIMENSION Z(1430) |
|
READ(10,100) D,NCOL,NROW,XO,DELX,YO,DELY |
100 |
FORMAT (A64, 2I5, 2(F6.0, F2.0), 7062X |
|
DO 10 I=1,NCOL |
|
READ (10,200) (Z(J),J=1,NROW) |
200 |
FORMAT (1430F5.1,2X) |
10 |
CONTINUE |
|
STOP |
|
END |
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