Sometimes wgrib2 does not know the locations of the grid points. Perhaps
the wgrib2 code needs to be updated with routines to calculate the locations
of the grid points. Perhaps the grid is unstructured and the locations are
given on some web pages. The option, -grid_def, is
for these cases. If the field has a the name LAUV, LAPP or NLAT, the
field is copied to the lat variable within wgrib2. If the field is named
LOUV, LOPP or ELON, the field is copied ot the lon variable within wgrib2.
Once the lat and lon arrays are defined, wgrib2 then knows the location
of the grid points. There are two caveats. First, the lat and lon arrays are
erased on detecting a new grid definition. Second, wgrib2 normally works in
double precision for the lat/lon calculations. By obtaining the lat/lon from
the grib data, you are working in single precision.
$ wgrib2 IN.grb -grib_def (rest of command line)