In order to integrate quantitative data, you need to know (a) if units are commensurate, and (b) if so, how to do conversions. The Quantities, Units, Dimensions, and Types (QUDT) ontology serves three major purposes. First, it provides a global reference for units via URIs; this helps avoid tacit conventions that are prone to misinterpretation. Second, it provides for dimensional analysis via so-called “quantity kind” dimensional vectors;