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Using QUDT and OM in the FAIR² Specification

Introduction

This document provides guidance on how QUDT (Quantities, Units, Dimensions, and Data Types) and OM (Ontology of Units of Measure) are integrated within the FAIR² (FAIR SQUARED) framework to ensure semantic interoperability of measurement units in AI-ready datasets.

Why QUDT and OM Are Used in FAIR²

FAIR² builds upon the FAIR principles to enhance scientific datasets with context-rich metadata, AI-ready design, and Responsible AI alignment. Measurement units are critical for FAIR compliance, and both QUDT and OM provide robust RDF/OWL representations for handling units in a standardized way.

QUDT and OM in FAIR²

Feature QUDT OM
Primary Focus Engineering, aerospace, industry, geospatial Scientific precision, research
RDF/OWL Compatible ✅ Yes ✅ Yes
Unit Conversions ✅ Yes (via qudt:conversionMultiplier) ✅ Yes (via om:hasConversionFactor)
Temperature Conversions (e.g., F → C) ❌ No ✅ Yes (om:hasOffset)
Complex Derived Units (e.g., m/s²) ❌ No ✅ Yes (om:derivedFromUnit)
Industry Adoption NASA, OGC, IoT, digital twins Research, EOSC

How FAIR² Uses QUDT and OM

1. Representing Quantities and Units

FAIR² integrates QUDT and OM to ensure standardized unit representation in RDF/OWL. Below are examples of how they are applied in FAIR².

QUDT Representation

{
  "@context": {
    "qudt": "http://qudt.org/schema/qudt/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "ex": "http://example.org/"
  },
  "@id": "ex:measurement1",
  "@type": "qudt:QuantityValue",
  "qudt:numericValue": {
    "@value": "1000.0",
    "@type": "xsd:double"
  },
  "qudt:unit": {
    "@id": "qudt:Meter"
  }
}

OM Representation

{
  "@context": {
    "om": "http://www.ontology-of-units-of-measure.org/resource/om-2/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "ex": "http://example.org/"
  },
  "@id": "ex:measurement2",
  "@type": "om:Measurement",
  "om:hasValue": {
    "@value": "1000.0",
    "@type": "xsd:double"
  },
  "om:hasUnit": {
    "@id": "om:Meter"
  }
}

2. Ensuring Unit Consistency

When using both QUDT and OM, FAIR² ensures that units are consistently mapped:

{
  "@context": {
    "qudt": "http://qudt.org/schema/qudt/",
    "om": "http://www.ontology-of-units-of-measure.org/resource/om-2/",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "ex": "http://example.org/"
  },
  "@id": "ex:measurement3",
  "@type": ["qudt:QuantityValue", "om:Measurement"],
  "qudt:numericValue": {
    "@value": "1.0",
    "@type": "xsd:double"
  },
  "qudt:unit": {
    "@id": "qudt:Kilometer"
  },
  "om:hasValue": {
    "@value": "1000.0",
    "@type": "xsd:double"
  },
  "om:hasUnit": {
    "@id": "om:Meter"
  }
}

3. Handling Unit Conversions

QUDT-based conversion

{
  "@context": {
    "qudt": "http://qudt.org/schema/qudt/",
    "xsd": "http://www.w3.org/2001/XMLSchema#"
  },
  "@id": "qudt:Kilometer",
  "@type": "qudt:Unit",
  "qudt:conversionMultiplier": {
    "@value": "1000.0",
    "@type": "xsd:double"
  }
}

OM-based conversion (explicit conversion factors)

{
  "@context": {
    "om": "http://www.ontology-of-units-of-measure.org/resource/om-2/",
    "xsd": "http://www.w3.org/2001/XMLSchema#"
  },
  "@id": "om:Kilometer",
  "@type": "om:Unit",
  "om:hasConversionFactor": {
    "@type": "om:ConversionFactor",
    "om:hasNumericalValue": {
      "@value": "1000.0",
      "@type": "xsd:double"
    },
    "om:hasUnit": {
      "@id": "om:Meter"
    }
  }
}

4. Validating Units with SHACL

FAIR² employs SHACL validation to enforce unit correctness and conversions. Below is a SHACL shape for validating unit consistency between QUDT and OM.

{
  "@context": {
    "sh": "http://www.w3.org/ns/shacl#",
    "qudt": "http://qudt.org/schema/qudt/",
    "om": "http://www.ontology-of-units-of-measure.org/resource/om-2/",
    "ex": "http://example.org/"
  },
  "@id": "ex:UnitConsistencyShape",
  "@type": "sh:NodeShape",
  "sh:targetClass": {
    "@id": "qudt:Unit"
  },
  "sh:property": {
    "sh:path": {
      "@id": "om:hasUnit"
    },
    "sh:class": {
      "@id": "om:Unit"
    },
    "sh:maxCount": 1,
    "sh:message": "If an OM unit is used alongside a QUDT unit, they should be equivalent."
  }
}

Best Practices for FAIR² Integration

  1. Use QUDT for broad interoperability, especially in industry, geospatial, and IoT applications.
  2. Use OM for scientific and research-based measurements, particularly biomedical and physical sciences.
  3. Ensure unit consistency when using both ontologies in the same dataset.
  4. Validate unit correctness with SHACL to ensure data quality.
  5. Adopt conversion multipliers carefully, using qudt:conversionMultiplier for simple unit scaling and om:hasConversionFactor for complex derivations.

Conclusion

FAIR² integrates QUDT and OM to enhance machine-readability, semantic clarity, and interoperability of datasets. By enforcing SHACL validation and ensuring accurate unit representation, FAIR² supports AI-ready, FAIR-compliant datasets across multiple domains.