<DNAG GRAVITY, 6 KM:>                               dnaggrav.doc
                          ............
                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:

1.  ID - 64 alphanumeric characters of identification.  The
identification is:  6 KM GRIDDED GRAVITY DATA OF NORTH AMERICA,
recorded in A64 format.

2.  NCOL - The number of columns of the grid.  This value is 1495
and is recorded in I5 format.

3.  NROW - The number of rows of the grid.  This value is 1430 and
is recorded in I5 format.


4.  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.

5.  DELX - The distance in kilometers between the columns of the grid.
This value is 6. and is recorded in F2.0 format.

6.  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.

7.  DELY - The distance in kilometers between the rows of the grid.
The value is 6. and is recorded in F2.0 format.

8.  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) ID,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