Type	Language / syntactical evaluation of unit expressions
Coverage	Unlimited / is able to express any imaginable unit of measure
Hosted by	Regenstrief Institute
Homepage	https://ucum.org

UCUM (Unified Code for Units of Measure) can represent any unit used in science, engineering a business for electronic communication. It provides a single coding system for units that is complete, free of all ambiguities, and that assigns to each defined unit a concise semantics.

UCUM makes use of atomic expressions representing well known base units and a syntax used to combine these atoms to more complex units if needed.

The SI/ISQ as well as other unit systems system allows an unlimited number of possible unit expressions for a single unit. UCUM is able to clearly identify a unit no matter what unit expression is used for it.

UCUM is a recognized standard in a wide field of applications and is recommended or incorporated by other standards for example the medical field (HL7, DICOM, ISO 11240), or geosciences (WMS, GML)

UCUM defines

• atomic symbols for the 7 base units of the SI / ISQ System
• symbols for prefixes used with atomic units
• a large list of symbols used for named, atomic, non-SI units
• a syntax description defining the rules used to combine the defined atomic symbols to express complex units

Type	Ontology
Coverage	Limited / around 1750 Units originating from 10 systems of units for 880 quantity kinds
Hosted by	public charity non-profit organization
Homepage	https://QUDT.org https://github.com/qudt/qudt-public-repo

The QUDT ontology includes

• physical constants, quantity kinds, units, unit systems, prefixes and dimension vectors
• a semantic description of the named entries including the relation to other entries
• translations to other standards of unit description

QUDT is modelled in OWL (Web Ontology Language). Other descriptions and interfaces are available like RDF/XML, TURTLE, JSON or SPARQL.

QUDT not only focuses on defining models and a publicly available vocabulary to express quantities and units but also wants to offer a translation between existing standards doing the same. Therefore, the ontology also names UCUM, UNECE, IEC 61360 and LaTeX codes for the entries.

QUDT originated from a NASA project trying to define a semantic specification for units of measure, quantity kind and dimensions used in science and engineering. Today it is governed by a board of directors from different research institutes and the industry.

Type	Dictionary
Coverage	A single list of code elements for units of measure for use worldwide in administration, commerce, transport, science and technology.
Hosted by	The United Nations, through its Centre for Trade Facilitation and Electronic Business (UN/CEFACT).
Homepage	https://www.unece.org/cefact/

The United Nations Economic Commission for Europe through its UN Centre for Trade Facilitation and Electronic Business (https://www.unece.org/cefact/), develops, maintains and publishes for free of charge a number of code lists.

Recommendation 20 provides three character alphabetic and alphanumeric codes for representing units of measurement for length, area, volume/capacity, mass (weight), time, and other quantities used in international trade. The codes are intended for use in manual and/or automated systems for the exchange of information in administration, commerce, transport, science and technology.

The code list (see https://www.unece.org/cefact/codesfortrade/codes_index.html) is presented in three separate annexes:

Annex I – Code elements listed by quantity category;

Annex II – Code elements listed by unit of measure name; and

Annex III – Code elements listed by common code.

See 5.6.3.4 how this information is mapped to Properties of DataType EUInformation.

Type	Dictionary
Coverage	A registry for metadata used for classification and description of products in all industrial/technical domains.
Hosted by	International Electrotechnical Commission (IEC)
Homepage	https://cdd.iec.ch/

The IEC Common Data Dictionary (CDD) is a registry for metadata used for classification and description of products in all industrial/technical domains.

The data model is described in IEC 61360. The dictionary includes quantities and units that are described in IEC 62720 - Identification of units of measurement for computer-based processing.

It covers any standard or non-standard units of measure currently in use, in two or more distinct ethno-linguistic groups or nations, at least in one domain of industry, for which an explicit method of conversion to a known standard unit of measure or its equivalent is well documented or evident from external references.

Type	Typesetting Rules
Coverage	Limited / Focuses on the SI units and provides rules to create complex units
Hosted by	Joseph Wright
Homepage	siunitx is free at https://ctan.org/pkg/siunitx https://www.texdev.net/

The siunitx package is a set of tools for the software system LaTeX. It is used to typeset pysical quantities from the SI. The package has an extended set of configuration options which make it possible to follow varying typographic conventions with the same input syntax.

______________