Re: [ferret_users] missing values and scale factors interfere?

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi Andrea,
You're right!  Thank you for the report. The CF standard, 
http://www.cgd.ucar.edu/cms/eaton/cf-metadata/CF-1.0.html, which we use 
for NetCDF files, has this paragraph

   The missing values of a variable with scale_factor and/or add_offset
   attributes (see section 8.1
   <http://www.cgd.ucar.edu/cms/eaton/cf-metadata/CF-1.0.html#pack>)
   are interpreted relative to the variable's external values, i.e.,
   the values stored in the netCDF file. Applications that process
   variables that have attributes to indicate both a transformation
   (via a scale and/or offset) and missing values should first check
   that a data value is valid, and then apply the transformation. Note
   that values that are identified as missing should not be transformed.

Ferret does save the correct value as the missing-value flag (say 
`temanom,return=bad` does return the -9999 missing value), but when it 
applies the the scale and offset, it does not check for the missing 
value flag before rescaling. This bug will be fixed in the next release. 
You're correct that the workaround for the time being is to

 set ver/bad =-99.99 my_var

but of course that is not a good solution if a variable might contain valid values of -99.99!

Ansley

Grant Andrea Nicole wrote:

> Hi,
>
> I have a netcdf file of the CRUTem2v global surface tempertures from the Climatic 
> Research Unit (http://www.cru.uea.ac.uk/cru/data/temperature/).
> In the attributes the 'scale_factor' is set to 0.01 and 
> 'missing_value' is set to -9999. In the file, the raw data indeed contains many
> -9999. However, when I read it into ferret, these all become -99.99 after the
> scale factor is applied and when I then try to do a shade command the 
> colorbar scale is worthless because so many -99s are plotted. It is easy
> enough to work around by defining
>
> set ver/bad =-99.99 my_var
>
> but I'm wondering if this is something that could be fixed? The other programs I've
> used (grads or matlab) are able to interpret this combination correctly. Or is
> it that the header should be defined differently, but possibly then other software
> wouldn't interpret it correctly? I am new to gridded data and not sure if there is
> a universal convention about these sorts of things.
>
> Thanks,
>
> Andrea
>
>
> Andrea Grant 
> Institut für Atmosphäre und Klima 
> ETH Zürich 
> Universitätsstrasse 16 
> CH-8092 Zürich 
> Schweiz 
> Tel: +41 (0)44 632 79 75 
> andrea.grant@env.ethz.ch
>
>