canonical units links

[carueda]

The idea here is to provide link(s) from this field to the associated units definition(s) in some semantic repository.

In general, we would probably need to parse the string according to the unit grammar to properly extract the relevant components for a comprehensive association of links. However, unless there is already some Javascript library for this purpose, we can simply tokenize the string and provide links only for the atomic components that have corresponding symbols in the UDUNITS vocabularies.

Refs:

• https://github.com/graybealski/cf-conventions-work/blob/master/FAQ.md#how-do-i-specify-units-in-cf
• http://www.unidata.ucar.edu/software/udunits/udunits-2.2.16/doc/udunits/udunits2lib.html#Examples