G2DRT
 
     G2DRT specifies the scaling and packing options when encoding the
     GRIB2 message:
 
        DRT number | decimal scale factor | binary scale factor | diff order |
        Missing value Management | Lossy compression ratio
 
     The DRT number is the Data Representation Template number used to pack 
     the data field.  Current valid values are:
          0 = Simple Packing (default)
          2 = Group Packing
          3 = Group Packing w/ differencing
         40 = JPEG2000 encoding
         41 = PNG encoding

     All data values are multiplied by 10^n, where n is the decimal scale 
     factor.  The default value is n=0.

     The data values can also be scaled by dividing each value by 2^m, 
     where m is the binary scale factor.  The default value is m=0.
 
     The differencing order specifies how many times the difference of adjacent
     data values is calculated, and it can be set to 1 or 2.  This option is
     only valid when DRT number 3 is specified.  The default diff order is 1.

     The missing value management flag determines whether missing values 
     are packed along with the data or an appropriate bitmap is constructed.
     A value of 1, use missing value management, is only valid for DRT
     numbers 2 and 3.  A value of 0, use bitmap when needed, is the default.

     A target lossy compression ratio can be specified only when using DRT 
     number 40.  A value of m informs the JPEG2000 encoding algorithm to
     attempt to compress to a ratio of m:1.  The default value of 255 
     indicates only lossless compression should be used.