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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