Re: [ferret_users] Missing value flag.

[Jaison Kurian <jaison@xxxxxxxxxxxxxxxxxx>]

Hi Nikesh,
            The trick is to define a "new" variable and assign the missing_value
as NaN. For those variables loaded from NetCDF file, the "set var/bad=NaN" command
will change the missing_value only in a temporal manner (see Ferret User Manual
Ref Sec31.11.1.  SET VARIABLE/BAD=). The following example worked fine for me 
(with Ferret versions 5.53 to 6.00):

    use coads_climatology
    let sfc_temp = sst[d=1]
    set var/bad=NaN/title="My Temp"/units="^oC" sfc_temp
    sp rm -f junk.nc
    save/file=junk.nc/append sfc_temp

Now, do a ncdump on junk.nc, it will show the missing_value (and _FillValue)
attribute as 

        float SFC_TEMP(TIME, COADSY, COADSX) ;
                SFC_TEMP:missing_value = nanf ;
                SFC_TEMP:_FillValue = nanf ;

and the data values for sfc_temp will be like :

 SFC_TEMP =
  nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
    nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,
    ......................................................................
   -1.56, -1.448571, -1.147778, -1.242857, -1.131111, -1.11, -1.1125,
    -0.8677778, -0.675, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan,

You can also set the missing_values to NaN in Matlab itself. Typically, 
misssing_values will be large -ve values. In Matlabe use an IF construct
to convert all missing values to NaN like :

    if (var < -1000)
         var = NAN
    end

Relevant Topics on Ferret User Manual 

   Ch3 Sec1.6.1.  Missing values in input files 
   Ref Sec31.11.1.  SET VARIABLE/BAD=  

Hope this helps,

Jaison


On Mon, 6 Aug 2007 nikesh@xxxxxxxxxxxxxxxxxxxx wrote:

> Hi ferreters,
> 
>  I want to change the missing value flag in a netcdf file from the default value
> to NaN, which is compatable with matlab.
> 
> I have tried the
> 
>  set var/bad=NaN but it only changes the missing value for the current ferret
> session, if I am writing a new netcdf file even after the set var/bad=NaN
> command, in new netcdf file also the default value is given. Should I do
> something else for attaining that.
> 
> Thanks in advance.
> 
> 

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