Creation of Raster GIS compatible files
   
   The following summary documents the sequence of steps accomplished by
   NGDC staff to transform the original DMAG & DGRAV binary files(from the
   Geophysics of North America CD-ROM, 1989), from raw data form into
   Raster GIS readable form.
   
   
  1. The IDRISI32 program was used to open the raster/grid file.
              DMAG & DGRAV  (.rdc & .rst)
   
  2. The IDRISI32 program was then used to create binary palette (.smp) files.

     - for Gravity, the display range was set between -362 to 125 mgals
     - for Magnetic, the display range was set between -1828 to 1716
       nanoTesla
     [Color assignments are made by a linear stretch in these ranges]
   
  3. For user access, these .smp files were converted to ASCII (.pal) form.
   
   The resulting files can be used in various GIS software packages
   including IDRISI32, GRASS, ERDAS and Adobe PhotoShop.

   For the Adobe PhotoShop software package, rename the .rst file to .raw,
   then open it with PhotoShop.  Tell PhotoShop the number of columns and rows
   in the file (contained in the .rdc file), and the data type (16-bit integer
   binary).

   The files can also be converted to other software formats.  The dgrav.rst
   image is an integer(16-bit), binary grid(2-dimensional array), with
   dimensions and locational details described in IDRISI header file dgrav.rdc.
   The .rst file extension may be changed to .img for ERDAS Imagine or earlier
   versions of IDRISI, to .bsq or .bil for ArcView, or to .raw for
   PhotoShop, etc., for reading or import of data.
   

NOTE:
   .rdc - ASCII system file containing run-time metadata / header information.
   
   .rst - binary raster data file in remote-sensing "image" strucuure.
          Data type and structure are documented in the .rdc file.

Reference:
   Gravity Anomaly Map Committee, 1987.  Gravity Amomaly Map of North
   America(1:5,000,000).  Boulder, Colorado, Geological Society of America,
   Continent-Scale Map-002(5 sheets).

   Geophysics of North America CD-ROM, National Geophysical Data Center, 1989.