GIS Transform  - (How these shapefiles were created.)
   
   The following summary documents the sequence of steps accomplished by
   NGDC staff to transform the original DMAG & DGRAV (binary) files from
   raw data form into GIS readable form.
   
   Step 
   
     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
        files (.smp)
                  - 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.
   
          4.  Next, the raster files were contoured producing vector line
        files.
                            DMAGC & DGRAVC
        - Contoured dgrav @ 20 mgal, intervals from -380 to 340
        - Contoured dmag  @ 100 nT, intervals from -1828 to 1716  
   
          5.  Still using IDRISI32, the resulting vector files (DMAGC &
        DGRAVC) were exported into a SHAPE file set.
                            DMAGS & DGRAVS
             - Three files (.shp, .dbf, .shx) were created for each data   
        type (gravity and magnetic).  
                       [NOTE: Three files (.shp, .dbf, .shx) constitute a shape
             file set.]
   
     6.  The above shape files were next opened with  ARCVIEW 3.1 to 
        create a custom palette.
   
   The resulting files can be used in various GIS software packages
   including ESRI's ARCVIEW 3.x and IDRISI32, and can be easily
   converted to other software formats.
                 _ _ _ _ _ _ _ _ _ _ _ _
   .rdc  - ascii system file containing run-time meta data / header.
   
        .rst   - binary raster data file in remote-sensing 'image' structure. 
        Data type and structure are documented in the .rdc file.
   
   .shp - the file that stores the feature geometry.
   
   .shx - the file that stores the index of the feature geometry.
   
   .dbf - the dBASE file that stores the attribute information of
   features.  When a shapefile is added as a theme to a view, this file
   is displayed as a feature table.