What's in a number? - xml-based repository of numerical metadata (edn.com)

(paraphrased fta): A quantity actually includes several components, Razdow explained. There are digits, obviously. Equally obviously to engineers, but often forgotten by the untrained, a quantity includes dimension in some defined units, and a tolerance. And in some cases, such as accepted constants also may have a proper name, like pi or Planck's constant. Attached to the quantity, either implicitly or explicitly, there is also a description, like minimal channel length or oxide thickness. Less obviously, the quantity may also have a history. Razdow's idea is to capture all these facets of a quantity in a single repository.