| 47 | | Trac is brought to you by [http://www.edgewall.com/ Edgewall Software], |
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| | 32 | ISO 2955 and all standards that do only look for the resolutions and recommendations of the CGPM and the Comité International des Poids et Mesures (CIPM) as published by the Bureau International des Poids et Mesures (BIPM) and various ISO standards (ISO 1000 and ISO 31) fail to recognize that the needs in practice are often different from the ideal propositions of the CGPM. Although not allowed by the CGPM and related ISO standards, many other units are used in international sciences, healthcare, engineering, and business, both meaningfully and some units of questionable meaning. A coding system that is to be useful in practice must cover the requirements and habits of the practice---even some of the bad habits. |
| | 33 | |
| | 34 | None of the current standards attempt to specify a semantics of units that can be deployed in information systems with moderate requirements. Metrological standards such as those published by the BIPM are dedicated to maximal scientific correctness of reproducible definitions of units. These definitions make sense only to human specialists and can hardly be deployed to their full extent by any information system that is not dedicated to metrology. On the other hand, ISO 2955 and ANSI X3.50 provide no semantics at all for the codes they define. |
| | 35 | |
| | 36 | The Unified Code for Units of Measure intends to provide a single coding system for units that is complete, free of all ambiguities, and that assigns to each defined unit a concise semantics. In communication it is not only important that all communicating parties have the same repertoir of signs, but also that all attach the same meaning to the signals they exchange. The common meaning must be computationally verifiable. The Unified Code for Units of Measure assumes a semantics for units based on dimensional analysis.2 |
| | 37 | |
| | 38 | In short, each unit is defined relative to a system of base units by a numeric factor and a vector of exponents by which the base units contribute to the unit to be defined. Although we can reflect all the meaning of units covered by dimensional analysis with this vector notation, the following tables do not show these vectors. One reason is that the vectors depend on the base system chosen and even on the ordering of the base units. The other reason is that these vectors are hard to understand to human readers while they can be easily derived computationally. Therefore we define new unit symbols using algebraic terms of other units. Those algebraic terms are also valid codes of The Unified Code for Units of Measure. |
| | 39 | |
| | 40 | == What is available? == |
| | 41 | |
| | 42 | The Unified Code for Units of Measures is very stable in content and has already been adopted by some standard organizations such as DICOM, HL7 and has been referenced as best practice by the Open Geospatial Consortium in their Web Map Service (WMS) and Geography Markup Language (GML) implementation specifications. We are still looking for the best way to establish this specification as a widely used industry standard. The official status and the affiliation may change during that process. However, we try to keep as much as possible of the specification freely available and redistributable to assure the maximum use and benefit. We would also like to keep this specification maintainable and flexible to updates. Although the initial version contains more than 250 terminal unit symbols (more than three times as many symbols as in ANSI X3.50), there are areas that are not covered completely yet. |
| | 43 | |
| | 44 | The specification is maintained electronically so that the printed version is guaranteed to contain consistent and tested data that is free from severe name conflicts or random errors. The full specification is now available as an HTML document (whereas it used to be only a PDF file). The PDF is still here for your convenience, however, the PDF is not updated at this time, hence it will slowly be outdated. The new XML format of the specification will soon enable us to make XML releases of the formal part of the specification, have better sorting and indexing capabilities, etc. |
| | 45 | |
| | 46 | A computer-readable XML file is now available which contains all the essential definitions. |
| | 47 | |
| | 48 | == IMPORTANT SPECIAL RELEASE NOTE == |
| | 49 | |
| | 50 | Version 1.6, November 2005 |
| | 51 | |
| | 52 | Ever since we changed the internal maintenance of The Unified Code for Units of Measure to XML (which happened after release 1.4 in May of 2002) the definition of some units that used exponential notation for the magnitude was incorrect. A systematic text-conversion error had caused the minus sign and the first digit of all exponents to be deleted. This mostly affected natural constants, such as parsec, proton mass, electron charge, Boltzman's constant, and all units that are defined based on these. Fortunately these units are rather rare in everyday use in trade and medicine, however, we must urge everyone to regenerate their tables, dictionaries, and knowledge bases, and check their data immediately based on this corrected release of The Unified Code for Units of Measure. |
| | 53 | |
| | 54 | The units directly affected by this error were: unified atomic mass unit (u), parsec (pc), Planck's constant [h], Boltzman's constant [k], electric permittivity ([eps_0]), elementary charge ([e]), electron mass ([m_e]), proton mass ([m_p]), Newton's constant of gravitation ([G]), Maxwell (Mx), Gauss (G), phot (ph), Curie (Ci), Roentgen (R), and U.S. and international mil ([mil_i], [mil_us]). |
| | 55 | |
| | 56 | Because these units were used in some (though not many) definitions of other units, the indirectly affected units are: electron volt (eV), and circular mil ([cml_i]). |
| | 57 | |
| | 58 | == More == |
| | 59 | |
| | 60 | There is an open-source reference implementation, instantly usable as a Java applet, that is configured at runtime over the Internet with the latest release of The Unified Code for Units of Measures. |
| | 61 | |
| | 62 | ---------------------------------------------- |
| | 63 | |
| | 64 | 1) Interestingly the authors of ENV 12435 forgot to include superscripts in the minimum requirements as given by subclause 7.1.4 for which they do not specify an alternative. |
| | 65 | |
| | 66 | 2)A more extensive introduction into this semantics of units can be found in: Schadow G, !McDonald CJ et al: Units of Measure in Clinical Information Systems. JAMIA. 6(2); Mar/Apr 1999; p.~151--162. Available from: URL: http://www.jamia.org/cgi/reprint/6/2/151 |
