Dear all,
I sort of agree with John on the "softness" of the current definition.
It actually came as a surprise to me to read that standard names
explicitly define the positive direction. I have never read the CF specs
that way.
The whole discussion indicates that there is no clear "standard
interpretation" for the positive attribute at this point.
I think this is a problem in itself - at least as far as automated
processing of data goes. Currently, I cannot see a machine-readable way
of determining the positive direction of a vertical coordinate from the
standard name table. Therefore I suggest to explicitly add this
information to the relevant standard names. In the XML file this could
be a separate attribute or tag in the data structure.
Example:
<entry id="depth">
<canonical_units>m</canonical_units>
<grib></grib>
<amip></amip>
<description>Depth is the vertical distance below the surface
</description>
</entry>
could be come something like:
<entry id="depth">
<canonical_units>m</canonical_units>
----> <positive>down</positive> <-----###########
<grib></grib>
<amip></amip>
<description>Depth is the vertical distance below the
surface.</description>
</entry>
An alternative specification of the positive direction could still be
supplied in a data file via the positive attribute if the CF specs make
it clear that the value in the standard name table is simply the default
and could be overwritten by a specific coordinate variable attribute.
Regards,
Michael
On 01/29/2014 06:40 PM, John Graybeal wrote:
> it will be interesting to see if/how others on the list weigh in. I agree with Jonathan on the implication of the words, and the importance of people using the standard correctly. At the same time, the CF Standard Names specification language is 'soft' -- I do not recall an explicit statement in the standard name definition (or anywhere else) that the axis units are always positive down when standard name is depth. (This ambiguity exists in other standard names as well.)
>
> John
>
> On Jan 29, 2014, at 09:26, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
>
>> Dear Steve
>>
>>> In practice I do not think that
>>> standard_name=depth and positive=up are necessarily in conflict (see
>>> bold text below). We fairly commonly encounter ocean model outputs
>>> in which the depths are encoded as negatives:
>>>
>>> 0
>>> -10
>>> -20
>>> ...
>>>
>>> There are merits to this encoding -- especially apparent in coupled
>>> ocean/atmosphere situations. (Also it preserves the right-handed
>>> coordinate system.) It would be quite peculiar (and a frequent
>>> source of error) to insist that an ocean model output file name its
>>> Z axis as "altitude", simply because it has encoded the depth values
>>> as negatives. The purpose of the 'positive' attribute is to inform
>>> the application when the encoding of values and the physical
>>> interpretation of the Z axis are reversed from one another.
>>
>> Maybe it's annoying, but I can't agree with that. I would say that such depth
>> values are wrong. The standard_name depth is defined as "the vertical distance
>> below the surface". -10 m means 10 m above the surface. There's no point in
>> having standard names unless we use them correctly, and this is one of the
>> kinds of imprecision standard names can help eliminate. It's fine for the ocean
>> model to encode -10 m for 10 m below the surface if it wants to, but then it
>> should use the standard_name of height instead (or altitude, since distance
>> above the surface and above the geoid is approx the same thing in ocean areas).
>> In all the ocean model data I work with (mostly from CMIP), depth is a positive
>> number, as it should be.
>>
>> Best wishes
>>
>> Jonathan
>> _______________________________________________
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>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
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>
--
Michael Decker
Forschungszentrum J?lich
Institut f?r Energie- und Klimaforschung - Troposph?re (IEK-8)
Tel.: +49 2461 61-3867
E-Mail: m.decker at fz-juelich.de
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