Re: Fill Value (fwd)

>From: Jason Burks <address@hidden>
>To: address@hidden
>Subject: Fill Value

Hi Jason,

> I am decoding the NOAA Experimental Wave Model (Ocean Waves) from grib 
> to netcdf. This model has missing values over land for obvious 
> reasons. If the variable in the cdl file FillValue and the value in 
> gbds.h define FILL_VAL are the same then the missing values get filled 
> in with a _ rather than the fill value. This would be ok except when 
> AWIPS sees this _ it tries to put the fill value in there and then 
> displays it. So what I need to know is there a way to modify the 
> gribtonc decoder to actually use the FillValue rather than put in the 
> underscore?
> I would not normally care but I need to have in the data file that the 
> fillvalue is -99999. and the missing data also has to show up as 
> -99999.

The "_" appearing for fill values is just the way the ncdump utility
shows a fill value.  The value stored in the netCDF file is still the
numeric value -99999. in your case, as you can verify by calling the
appropriate nc_get_var() C function or NF_GET_VAR()Fortran function.

If you want the ncdump utility to show the numeric value instead of
the "_" abbreviation for fill values, you would need to modify
ncdump/vardata.c and rebuild ncdump, by deleting some lines.

From the printbval, printsval, and printival functions, delete

    if (varp->has_fillval) {
        double fillval = varp->fillval;
        if(fillval == val) {
            (void) sprintf(sout, FILL_STRING);
            return;
        }
    }

From the printfval function, delete

    if(varp->has_fillval) {
        double fillval = varp->fillval;
        if((val > 0) == (fillval > 0) && /* prevents potential overflow */
           (absval(val - fillval) <= absval(float_eps * fillval))) {
            (void) sprintf(sout, FILL_STRING);
            return;
        }
    }

And from the printdval function, delete

    if(varp->has_fillval) {
        double fillval = varp->fillval;
        if((val > 0) == (fillval > 0) && /* prevents potential overflow */
           (absval(val - fillval) <= absval(double_eps * fillval))) {
            (void) sprintf(sout, FILL_STRING);
            return;
        }
    }

There probably should be a command line option on ncdump to not print
"_" for fill values so you wouldn't have to modify the code and
recompile, but no one ever requested such a feature.

--Russ
_____________________________________________________________________

Russ Rew                                         UCAR Unidata Program
address@hidden                     http://www.unidata.ucar.edu