Units of Measure Convention Metadata

• UUID: 3bbe438d-df37-49fe-8e2b-739296d46dfb
• Name: Units of Measure
• Schema URL: "https://raw.githubusercontent.com/clbarnes/zarr-convention-uom/refs/tags/v1/schema.json"
• Spec URL: "https://github.com/clbarnes/zarr-convention-uom/blob/v1/README.md"
• Scope: Array, Group
• Extension Maturity Classification: Pilot
• Owner: @clbarnes

Description

This convention defines how to communicate the units of the data contained in a numeric Zarr array. The license metadata exists in a field called uom at the root attributes level following the Zarr Conventions Specification.

Communicating the units of measure used by numeric is applicable across ecosystems and applications, and should be made as low-friction as possible to facilitate re-use of FAIR data.

Units MUST be defined on each array individually. Related arrays sharing a unit SHOULD validated at the application level.

• Examples:
  • Full metadata
  • Minimal metadata
  • Arbitrary units

Motivation

• Comparability: Units of measure are an essential piece of data for comparing raw data
• Not crashing your mars orbiter

Convention Registration

The convention must be registered in zarr_conventions:

{
  "zarr_conventions": [
    {
      "schema_url": "https://raw.githubusercontent.com/clbarnes/zarr-convention-uom/refs/tags/v1/schema.json",
      "spec_url": "https://github.com/clbarnes/zarr-convention-uom/blob/v1/README.md",
      "uuid": "3bbe438d-df37-49fe-8e2b-739296d46dfb",
      "name": "uom",
      "description": "Units of measurement for Zarr arrays"
    }
  ]
}

Applicable To

This convention can be used with these parts of the Zarr hierarchy:

• Group
• Array

Properties

The uom key at the root attributes level MUST be an object.

Convention metadata name: uom

Field Name	Required	Type	Description
uom	yes	UOM Object

Example:

{
  "attributes": {
    "zarr_conventions": [
      { "name": "uom", "uuid": "3bbe438d-df37-49fe-8e2b-739296d46dfb" }
    ],
    "uom": {
      "ucum": {
        "version": "2.2",
        "unit": "kg"
      },
      "description": "apple count as weight equivalent of one bushel"
    }
  }
}

UOM Object

Field Name	Required	Type	Description
ucum	yes	UCUM Object
description	no, default ""	Free text description of the measurement

UCUM Object

Field Name	Required	Type	Description
unit	no, default "1"	string	UCUM unit string
version	no, default most recent supported	string	Version of UCUM specification

"unit" (uom.ucum.unit)

This MUST be a valid case-sensitive unit according to the Unified Code for Units of Measure.

By default (field omitted or set to null) the unit MUST be considered arbitrary.

Known Implementations

Libraries and Tools

• zarrs_conventions_uom - zarrs / zarrs_conventions implementation
  • Language: Rust
  • Status: Experimental
  • Maintainer: @clbarnes
  • Since: 2026-01-07

Datasets Using This Convention

Resources

If you implement or use this convention, please add your implementation to this list by submitting a pull request.

Acknowledgements

This template is based on the STAC extensions template.