Hi Phil,
I think that's a misinterpretation of the Users Guide attribute
conventions. Under the description for _FillValue, the Guide states:
The fill value ... is normally outside the valid range and therefore
treated as missing when read by generic applications. It is legal (but not
recommended) for the fill value to be within the valid range.
The last sentence implies that the valid range is not determined by the
_Fill_Value.
I think it's intended that the rule about _Fill_Value under the description
for valid_range only applies in case none of valid_min, valid_max, or
valid_range are specified, so it wouldn't apply to your example:
If neither valid_min, valid_max nor valid_range is defined then generic
applications should define a valid range as follows. ...
However, I suspect that the rule is confusing enough that writers of
generic clients might just ignore it, even in that case.
--Russ
On Wed, Apr 15, 2015 at 8:55 AM, Bentley, Philip <
philip.bentley@xxxxxxxxxxxxxxxx> wrote:
> Hi folks,
>
> I was hoping someone can clarify for me the correct use of the valid_min,
> valid_max and valid_range attributes by *well-behaved* netCDF clients.
>
> Given the netCDF file shown below (in CDL form), and considering the rules
> for handling the aforementioned attributes, as defined in Appendix A of the
> NetCDF user guide (see
> http://www.unidata.ucar.edu/software/netcdf/docs/attribute_conventions.html),
> I *think* I'd expect conforming netCDF clients to represent the data for
> each of the variables var1..var3 as an array with values (_, -272, 0, 100,
> 9999, _), where _ signifies missing data values.
>
> ---
> netcdf mditest {
>
> dimensions:
> dim1 = 6;
>
> variables:
> float var1(dim1);
> var1:long_name = "var1";
> var1:units = "1";
> var1:valid_min = -273.0f;
> var1:valid_max = 1.0e5f;
>
> float var2(dim1);
> var2:long_name = "var2";
> var2:units = "1";
> var2:valid_min = -273.0f;
> var2:_FillValue = 1.0e5f; // constrains valid_max
>
> float var3(dim1);
> var3:long_name = "var3";
> var3:units = "1";
> var3:_FillValue = -273.0f; // constrains valid_min
> var3:valid_max = 1.0e5f;
>
> // global attributes
> :Conventions = "CF-1.0";
>
> data:
> var1 = -300, -272, 0, 100, 9999, 1e6 ;
> var2 = -300, -272, 0, 100, 9999, 1e6 ;
> var3 = -300, -272, 0, 100, 9999, 1e6 ;
> }
> ---
>
> However, having tried several different netCDF clients - some C, some
> Java, some Python - none of them appear to adhere consistently to the
> aforementioned rules for handling the valid min/max/range attributes. The
> python-based clients, in particular, only seem to honour the _FillValue
> attribute, reflecting, I believe, the current behaviour of the
> netcdf4-python module.
>
> Am I perhaps misinterpreting the nc attribute-handling conventions?
>
> Regards,
>
> --
> Phil Bentley PhD, Climate Science IT Apps Group
> Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB, United Kingdom
> Tel: +44 (0)1392 886881
> Email: philip.bentley@xxxxxxxxxxxxxxxx Web: http://www.metoffice.gov.uk/
>
>
> _______________________________________________
> netcdfgroup mailing list
> netcdfgroup@xxxxxxxxxxxxxxxx
> For list information or to unsubscribe, visit:
> http://www.unidata.ucar.edu/mailing_lists/
>
>
