[June 30, 2000] The 'Measurement Units Markup Language' effort is referenced on NIST's "MatML" web site. Apparently an unannounced work in progress.
Excerpt from the June 7, 1999 paper: "This paper discusses how the XML Schema Working Group's current Datatypes proposal can be extended to support structured representation of measurement units and related information. We outline an incremental approach to incorporate measurement units into XML and XML schemas which can integrate a variety of measurement units representations, from instance level units specifications, to detailed formal type specifications of underlying measurement properties, dimensions, coordinate systems. Time may not permit incorporation of the full proposal outlined here into the initial Proposed Recommendation for XML Datatypes. But our XML Schema Working Group can and should go beyond simple lexical representation of dates, times, and other common measurement units, as proposed in our current drafts. This paper shows how we can begin to do so by simply adding measurement units, etc. as optional facet(s) to the current XML Datatypes draft. We also provide a roadmap for more complete treatment of measurement units in Version 2.0 of the XML Datatypes Recommendation..."
"What XML Schema Designers Need to Know About Measurement Units." XTech 2000 Presentation by Frank Olken and John McCarthy (Lawrence Berkeley National Laboratory).