Re: [ferret_users] missing value not detected in double-precision ferret

[Russ Fiedler <russell.fiedler@xxxxxxxx>]

Hi Hella,

In your third example you tried to use the attribute "missing" to denote
the missing value. You really need to call this either "missing_value"
or "_FillValue" and Ferret will pick this up (along with other software
that recognises CF standard files).

Adding the missing value attribute with nco may or may not be a quick
task. If there isn't sufficient space in the definition part of the file
nco may have to make a temporary copy of each file. Still, in the long
term it will probably be worth it to have CF compliant files.

Russ

On Mon, 2013-06-10 at 23:13 +0200, Hella Riede wrote:
> Dear ferret list,
> 
> there is a problem reading our climate model netcdf files with the new
> ferret double precision version. I boiled it down to a set of small
> files in the attachment.
> 
> '-1E+34' is not recognized as 'missing value' (see 'ex_float'). To save
> space, all variables except dimensional variables in our climate model
> output are defined as "float" = "single precision".
> 
> When a netcdf file is read into double-precision ferret, the entry
> '-1E+34' in a float (single precision) variable is not recognized as
> 'missing value' anymore, but has a numerical value of
> -9.999999790214768E+33 (prec=16). The bad values are thus not masked,
> which is inconvenient for plotting or analysis.
> 
> yes? list press
>                  VARIABLE : pressure (Pa)
>                  FILENAME : ex_float.nc
>                  SUBSET   : 3 points (TIME)
>      01-JAN-2000 00:20:00 / 1:  1.000E+04
>      01-JAN-2000 00:40:00 / 2: -1.000E+34
>      01-JAN-2000 01:00:00 / 3:  1.000E+05
> 
> The workaround 'set var/bad=-9.999999790214768E+33
> <variable>' is inconvenient as well.
> 
> Interestingly, when listing the bad value of the variable, the bad-value
> masking seems to work:
> 
> yes? list `press,ret=bad`
>      !-> list -1.E+34
>                  VARIABLE : constant
>             ....
> 
> In 'ex_double' (attachment), I exchanged 'float' to 'double' by hand for
> all variables. The value '-1E+34' is then recognized correctly:
> 
> yes? list press
>                  VARIABLE : pressure (Pa)
>                  FILENAME : ex_traj2.nc
>                  SUBSET   : 3 points (TIME)
>      01-JAN-2000 00:20:00 / 1:   10000.
>      01-JAN-2000 00:40:00 / 2:     ....
>      01-JAN-2000 01:00:00 / 3:  100020.
> 
> The explicit definition of a missing value in the netCDF file does not
> solve the issue (attachment: ex_float_missing).
> 
> Please help us with backwards compatibility as we cannot go into several
> terabyte of climate data output to change "float" to "double" in the
> variable definitions.
> 
> 
> Thanks a lot in advance,
> Hella
> 
>