Statement of purpose:
Ontologies allow the explicit specification of the multiple possible meanings of concepts so that people can recognize commonalities and differences in the semantics of the concepts that they use.
Ontologies can be used to improve the quality of standards, leading to more robust implementations of the standards and the semantic integration of multiple standards. The axiomatization of formal ontologies can also support automatic conformance-checking.
Measurement units and dimensions (or dimensionality) are essential for the meaningful communication of measurements, design specifications, scientific data, medical data, environmental data and regulations, and many commercial transactions. Confusion over measurement units can lead to disasters such as the demise of the Mars Climate Observer satellite. An ontology of measurement units and dimensions would have wide utility in many IT standards.
A number of standards projects and other large-scale projects are currently developing some kind of ontology for quantities and measurement units. This will quickly lead to a proliferation of formal models for quantities and units that are not quite comparable. That in turn will impede consistent specifications of quantities for publication and information exchange in many industries. A standard ontology for quantities and units, adopted at this time, can be incorporated into such projects, or used as a reference for the symbols they define, thus promoting consistent interpretation and interworking of specifications and measurements.
Measurement units include metres, feet, inches, etc. all of which have the dimension of "length", i.e., length is the "property" of which "metre" is the unit of measure. In the International System of Quantities (Units) (also called the metric system or SI) the base dimensions (units) are: length (metre), mass (kilogram), time (second), electric current (ampere), thermodynamic temperature (kelvin), amount of substance (mole) and luminous intensity (candela). Derived (or composite) dimensions are constructed by multiplying or dividing the dimensions when multiplying or dividing the corresponding quantities. Hence speed has dimension of length / time. In practice the various base dimensions may have exponents of -3 to +3. Thus the space of derived dimensions has size of 7 to the 7th power - approx. 800K possible dimensions. For each dimension there are often several alternative measurement units - thus the space of all possible measurement units is huge.
Thus is it is clear that there is need to specify a framework for constructing derived dimensions / units from base dimensions and units.
It is therefore proposed to develop an ontology which would specify the basic concepts of quantities, systems of quantities, and systems of measurement units and scales, the various base dimensions and units of the SI system, the various metric prefixes (nano-, micro-, milli-, kilo-, ...), the rules for constructing various derived units, and the designations of the most common derived units such as joules, watts, ... The ontology should also address non-metric base and derived units that are commonly used across multiple industries.
The ontology will be represented in multiple formats, to allow exploitation via various tools - CLIF (ISO 24707) is currently proposed as the reference normative form, along with a derived OWL2/DL representation, although this may not be able to accommodate all CLIF concepts. It will also be important to construct canonical URIs to reference the various measurement units and dimensions. Furthermore, the ontology should be linked to standard representations (names, abbreviations) for the various measurement units / dimensions, e.g., meters, m, joules, etc., as expressed in sources such as the OASIS UnitsML project, and the Healthcare Level 7 Uniform Code for Units of Measure (UCUM).
There are some anomalies which need to be addressed, notably in the area of "dimensionless units" and derived units that involve them, such as measurements of concentration, energy v. torque. Non-linear scales will also be a concern.
Scope of the work of the TC:
The objective is to develop the draft of an international ontology standard for expressing "Quantities and Units of Measure" which will be publicly available, free of charge. The reference normative form of the ontology will be expressed in the CLIF language, with derived normative representations in OWL 2.0 DL, and UML.
The work shall include the development of a core set of ontology modules covering quantities, units, scales, dimensions, base and derived SI units and their relationships, and extension mechanisms to allow the later inclusion of non SI units and other measurement systems. The ontology shall be independent of industry sector and applications, and based on available specifications and standards, particularly the VIM. Where ambiguities and contradictions arise between different specifications and standards, these shall be referred back to the originating organizations for resolution.
Later ontology modules may include basic concepts for measurements and uncertainties, and for quantity specifications and tolerances. While all applications of quantities are one or the other of these, these disciplines are more complex and may have industry-specific characteristics.
The TC will liaise and strive to coordinate its development with the work of BIPM (International Bureau of Weights and Measures), ISO/IEC 80000, VIM (International Vocabulary of Metrology), UnitsML, UCUM (Unified Code for Units of Measure) and UNECE Recommendation 20, and take into account existing quantities and units ontologies based on VIM, such as Sweet, QUDT and the QUDV component of SysML. It is expected that the scope of the QUOMOS ontology with respect to specific units will be influenced by the relationship to UnitsML and UCUM.
Consideration will be given to ensuring that the methodology used to construct the ontology can be exploited in the construction of other definitive ontologies.