As I read those docs, and as I've always interpreted netcdf, those "rules"
are there to inform the user of the data -- I don't expect client software
reading the data to replace values in the the arrays for me -- at least not
low-level generic data reading libraries.
What were you expecting, and why?
Higher level libs that represent a data model _may_ do some QA/QC on the
data, I suppose, but even then, I'd want to clearly specify what I wanted
it to do -- certainly not silently toss out values because the were outside
the limits specified in the meta-data.
-Chris
On Wed, Apr 15, 2015 at 9:18 AM, John Caron <caron@xxxxxxxx> wrote:
> The netcdf-java library does indeed ignore that rule. We should revisit
> the manual and clarify things i guess.
>
> On Wed, Apr 15, 2015 at 9:33 AM, Russ Rew <russ@xxxxxxxxxxxxxxxx> wrote:
>
>> 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/
>>>
>>>
>>
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>
>
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--
Christopher Barker, Ph.D.
Oceanographer
Emergency Response Division
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