[CF-metadata] "Andrew Clegg": udunits 1 or 2 for CF?

Andrew Clegg ancl at pml.ac.uk
Thu Mar 4 09:16:23 MST 2010


Steve,

I just tried adding u as a prefix, and it certainly worked (although I 
also can't say if it would have any unintended consequences, I only use 
a small set of unit strings).

I don't use Fortran personally, so a lack of a Fortran interface 
wouldn't be a problem for me. Of course, we would not want to 
deliberately cause problems for Fortran users. Jonathan's response 
reflects my opinion on this:

We can distinguish udunits-2 as a standard for units strings, and udunits-2
as software. If we say that CF uses udunits-2 as its reference, programs in
Fortran can still use udunits-1 to process the units strings, in nearly all
cases, though obviously a ununits-2 Fortran interface would be preferable.

Cheers,
Andrew Clegg

Steven Emmerson wrote:
> Andrew,
>
> The "u" prefix can be added to the UDUNITS-2 database by inserting the 
> line
>
>         <symbol>u</symbol>
>
> in the set of other "symbol" definitions for the value "1e-6" in the 
> database file "$(prefix)/share/udunits/udunits2-prefixes.xml".
>
> At this time, however, I can't say whether or not doing so will have 
> unintended consequences.  If you wish to try it, please do so and get 
> back to me.
>
> I think a larger issue regarding the adoption of the UDUNITS-2 package 
> is the fact that it doesn't yet have a Fortran interface.  I have not 
> investigated this matter much but, apparently, Fortran-95 has a 
> relatively easy mechanism for creating such an interface -- which, 
> hopefully, would mean that few resources would be needed to create 
> one.  The question then becomes one of priority: given that the CF 
> convention adopts the UDUNITS-2 package, how necessary or important is 
> a Fortran interface?  On the other hand, given that creating a Fortran 
> interface will consume resources (most especially time) how necessary 
> is adoption of the UDUNITS-2 package?
>
> I encourage and await your input on the matter.  Would you please CC 
> me directly as I don't (yet?) subscribe to the CF mailing-list.
>
> Regards,
> Steve Emmerson
> UDUNITS Developer
>
> On 3/4/2010 3:08 AM, Andrew Clegg wrote:
>> Steve,
>>> The UDUNITS-1 library had a bug in it. The bug was that "g" was
>>> interpreted as meaning "gravity" (i.e., "standard_free_fall") rather
>>> than "gram". This was corrected in version 1.12.10 of the UDUNITS-1
>>> package, which was released on 2009-09-17.
>>>
>>> The UDUNITS-2 package did not have this bug from the outset.
>> Thanks for this, however this is not the only incompatibility I have
>> found. Another one is specifying microns. Udunits1 understands 'um',
>> while Udunits2 recognises 'µm'. I note that the CF conventions list in
>> Table 3.1 that the abbreviation for micro is 'u', not 'µ'.
>>
>> It may well be that these are the only 2 incompatibilities - in which
>> case, it might be wise to note these specifically in the conventions, ie
>> to specify that "The value of the |units| attribute is a string that can
>> be recognized by UNIDATA"s Udunits package (version 1.12.10 or higher)
>> <http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/cf-conventions.html#udunits>", 
>>
>> and perhaps that '1e-6' should be used in place of u/µ to ensure
>> compatibility with both versions.
>>
>> Cheers,
>> Andrew
>>
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