Jonathan, Steve Emmerson wrote: > The problem with adding "B" or "Bel" to the UDUNITS database is that > logarithmic units always need a reference level and the grammar for > specifying units (which is used in the database) doesn't support it. > For example, the grammar needs to support something like > > dB (1 mW) > > for a decibel unit with a 1 milliwatt reference level. Although the syntax above is not currently supported by udunits, it's the syntax we intend to support when logarithmic units are added to udunits, within the next year. So it would be possible to use the above syntax now to make your intent clear, with assurance that eventually udunits will recognize it. I would also like to confirm Steve's comment that we intend to merge udunits into netCDF soon, so that users of both packages will be able to build and install them more easily. Steve also wrote: > One can always have one's own units database, but then God help someone > trying to do interdisciplinary work, or to have one's work immediately > understood years later. One possible solution to this sort of problem would be an attribute convention for specifying one or more URIs for replacement/additions/overrides to udunits.dat, analogous to XML namespaces. As part of merging udunits into netCDF, we would want to recommend such a convention, for example a "_Units" attribute, which of course would only be necessary to override the default that udunits is configured and installed with. Steve and I have discussed this and he's got some good ideas for implementing it. Another complication would be that the udunits library would have to support the possibility of multiple different units sets for simultaneously open netCDF datasets that specified different URIs. Of course eventually you would want to allow the use of XML to specify the units initialization information currently hardwired in udunits.dat, but that's a bit more than we can commit to right now and for backward compatibility we would need to support the current table syntax anyway. This might be something to discuss on the cf-metadata mailing list, but for now I've just Cc:ed the people who have been part of this thread and know the context ... --Russ _____________________________________________________________________ Russ Rew UCAR Unidata Program address@hidden http://www.unidata.ucar.edu/staff/russ
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