From the website:
The QUDT, or ‘Quantity, Unit, Dimension and Type’ collection of ontologies define base classes, properties, and instances for modeling physical quantities, units of measure, and their dimensions in various measurement systems. The goal of the QUDT collection of models is to provide a machine-processable approach for specifying measurable quantities, units for measuring different kinds of quantities, the numerical values of quantities in different units of measure and the data structures and data types used to store and manipulate these objects in software. A simple treatment of units is separated from a full dimensional treatment of units. Vocabulary graphs will be used to organize units for different disciplines.
Useful in a number of domains. Comparison to other measurement ontology efforts should prove to be interesting.