QUDTQUDT is a comprehensive OWL ontology that provides a standardized, machine-readable framework for representing quantities, units of measurement, quantity kinds, dimensions, and data types. With its quantity kinds vocabulary alone defining over 1,200 entries spanning physics, chemistry, engineering, and medicine, QUDT enables precise and interoperable representation of measurement data across scientific and engineering domains.
Background
QUDT was originally developed by TopQuadrant under contract with NASA to support consistent representation of engineering and scientific measurement data. The project was initiated around 2011 to address the challenge of integrating measurement data across NASA's diverse programs, where inconsistent unit representations could lead to costly errors. The ontology has since grown into a community-maintained open resource used well beyond its aerospace origins, reaching version 3.1.11 as of late 2024.
Purpose and Scope
QUDT addresses the fundamental problem of expressing measurements unambiguously in machine-readable form. It provides:
- Quantity Kinds: What is being measured (e.g., length, mass, temperature, force, absorbed dose, acceleration). Version 3.1.11 defines 1,214 quantity kinds.
- Units: The reference scale used (e.g., meter, kilogram, kelvin). The units vocabulary covers SI, CGS, imperial, and domain-specific scales.
- Dimensions: The dimensional analysis vector for physical quantities
- Data Types: Formal types for measurement values
- Conversion Factors: Enabling automated unit conversion
The applicable domain, as stated in the ontology metadata, is "Science, Medicine and Engineering" across "All disciplines."
Structure and Namespaces
QUDT uses a multi-graph architecture with distinct vocabularies:
| Namespace | Content |
|---|---|
http://qudt.org/schema/qudt/ |
Core ontology schema |
http://qudt.org/vocab/unit/ |
Units vocabulary |
http://qudt.org/vocab/quantitykind/ |
Quantity kinds vocabulary |
Each vocabulary graph carries its own metadata including version, creation date, contributors, and Turtle file URL.
Serializations
QUDT ontologies are available in Turtle (the primary distribution format), RDF/XML, and JSON-LD. HTML documentation is generated from the RDF for browsing. QUDT uses Dublin Core terms (dcterms) extensively for its own metadata.
Governance and Maintenance
Created by Ralph Hodgson and Steve Ray, with contributions from Jack Hodges and Simon J.D. Cox. The ontology is actively maintained by the QUDT.org community with frequent releases. It is distributed under a Creative Commons Attribution 4.0 International (CC BY 4.0) license with attribution to QUDT.org.
Notable Implementations
QUDT is used in scientific data management, manufacturing, engineering simulation, environmental monitoring, and linked data applications wherever precise unit and quantity representation is needed. Its comprehensive coverage of unit systems makes it a reference ontology for any system dealing with physical measurements in RDF.
Related Standards
- SKOS (skos): QUDT uses SKOS alignment properties (skos:closeMatch, skos:exactMatch) for vocabulary interoperability