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Summary

The World Meteorological Organization (WMO) Integrated Global Observing System (WIGOS) Metadata Representation (WMDR) code list for units provides standardized notation for the majority of units commonly used to measure Earth Science variables. The WMDR unit code list is an International System of Units (SI)-based unit system with a set of standardized notations that is comprehensive enough to support NASA Earth Science research activities in both the satellite and field campaign communities, covering observations and modeling analyses. WIGOS measurement units are readily adaptable to binary (e.g., NetCDF and HDF) and ASCII formats, and suitable to construct Uniform Resource Identifiers (URIs), which are necessary to enhance data product interoperability and (re)usability.

Status

The WMDR measurement unit code list was approved for use in NASA Earth Science Data Systems in July 2025.

ESCO RFCESDS-RFC-058 WIGOS Measurement Unit Standard
DOIhttps://doi.org/10.5067/DOC/ESCO/ESDS-RFC-058v1
Suggested CitationSilverman, M.L., Chen, G., Huffer, E.B., Buzanowicz, M.E., Collister, B., & Shook, M.A. (2025). WIGOS Measurement Unit Standard RFC. NASA Earth Science Data and Information Systems Standards Coordination Office. https://doi.org/10.5067/DOC/ESCO/ESDS-RFC-058v1.
SpecificationWMO Codes Registry, Register: Measurement unit

NASA Earth Science Community Recommendations for Use

Strengths

  • The WIGOS measurement unit code list offers comprehensive coverage for units commonly used to measure Earth Science variables.
  • Definitions for derived units and non-SI units include the SI base unit equivalent when applicable, providing a basis for interoperability with SI.
  • The measurement unit code list is maintained by a team of experts under an established governance process.
  • The measurement code list avoids special characters that are known to create machine readability problems.
  • Adopting the WIGOS measurement unit code notation will help improve interoperability and (re)usability of NASA Earth Science data.

Weaknesses

While it is believed that WIGOS units are consistent with UDUnits, which are used extensively for data packaged in NetCDF and HDF formats, this has not been tested.

  • There may be UDUnits that are not included in the WIGOS units, in which case a request to add the missing units would have to be submitted to add the unit to WIGOS, and there is a chance that such a request could be rejected.
  • Unidata, the creator of UDUnits, is not currently operating and compatibility testing with Unidata is infeasible at this time.

The existing governance process for WIGOS units is both a strength and a weakness. While the WIGOS unit codes are well-maintained by experts, that process is under the control of the WMO, and not NASA. Therefore, changes and updates to the code lists desired by NASA will have to be submitted to an external organization and NASA will not have decision-making authority over such requests. 

Applicability

WIGOS measurement units are applicable to and suitable for Earth science data produced by both satellite missions and field campaigns. 

Limitations

The adoption of the WIGOS Units standard is intended to apply only to the notation, and does not impose any requirements or even recommendations about which particular units should be used for particular types of measurements. Such decisions will remain in the hands of individual scientists or projects making the measurements and producing the data.