stnmap [ -i fname ]
stnmap is a GrADS utility  that  writes  out  a  hash
table
     and/or  link  list  information for station data that allows
     GrADS to access the report data more efficiently.
     
If you change the data file - perhaps by  appending  another
     time  group  -  you  will also have to change the descriptor
     file to reflect the changes - the new number  of  times  for
     example - and then rerun the stnmap utility.
     
Note the difference between station descriptor file and a grid descriptor file:
DTYPE                   specify a data
type of: station
STNMAP                  gives the file
name of the station mapping file.     
XDEF, YDEF, ZDEF        not
specified.
TDEF                    describes the
time grouping interval and
                         the  number  of  the  time groups in the
                         file.
VAR                     surface
variables must come first, and
                         are     given    a    zero    for    the
                         number-of-levels field. Level  dependent
                         variables  are  listed after the surface
                         variables,  and are given a one  in  the
                         number-of-levels field.
ua.reps  is  written,  and
the
     descriptor  file stat.ctl, including the information
records
     for the station data file ua.reps  is  created,  you
should
     then create the station map file ua.map (named in the
STNMAP
     record of the descriptor file; see  below)  by  running  the
     stnmap utility.
Example descriptor file: stat.ctl
 DSET   ^ua.reps
                    DTYPE   station
                    STNMAP ^ua.map
                    UNDEF   -999.0
                    TITLE   Real Time Upper air obs
                    TDEF    10 linear 12z18jan1992 12hr
                    VARS    12
                        slp   0  99  SLP
                        ts    0  99 Temps
                        us    0  99  U Winds
                        vs    0  99  V Winds
                        z     1  99  Heightsa
                        t     1  99  Temps
                        u     1  99  U Winds
                        v     1  99  V WInds
                    ENDVARS
Running the stnmap utility:
stnmap -i stat.ctl
     The station map file ua.map is a binary file, which
includes
     the hash table and/or link list information.