IFC 4.3.2.0 (IFC4X3_ADD2) official

4.1.9.9 Project Units

The project context includes the definition of the default units within the IFC data set. They are declared once for the context of the whole data set. Default units are those units that apply:

  • to all geometric representation items within the geometric representation contexts;
  • to all attributes with a defined datatype indicating a measure datatype;
  • to all properties and quantities with a defined datatype indicating a measure datatype and with no local unit definitions provided.

Default units are defined as either:

  • SI Units with SI unit name and prefix
  • Conversion based units with a conversion factor to its SI unit
  • Derived units, consisting of one-to-many unit elements, being SI units or conversion based units
  • Monetary unit as a special case of a unit to hold the currency name

The following diagram shows the generic classes and relationships used when applying this concept. In addition, concepts may have particular importance to common or standardised industry practices and scenarios. For these specific usage scenarios, the table below shows a recommended list of general usage patterns that users may adopt.

G IfcContext IfcContext 1. GlobalId [1:1] 2. OwnerHistory [0:1] 3. Name [0:1] 4. Description [0:1]      HasAssignments [0:?]      Nests [0:1]      IsNestedBy [0:?]      HasContext [0:1]      IsDecomposedBy [0:?]      Decomposes [0:1]      HasAssociations [0:?] 5. ObjectType [0:1] 6. LongName [0:1] 7. Phase [0:1] 8. RepresentationContexts [1:?] HasUnits 9. UnitsInContext [0:1]      IsDefinedBy [0:?]      Declares [0:?] IfcUnitAssignment IfcUnitAssignment 1. Units [1:?] IfcContext:UnitsInContext1->IfcUnitAssignment:IfcUnitAssignment0 IfcLabel_4 IfcLabel IfcContext:Phase1->IfcLabel_4:IfcLabel0 IfcLabel_5 IfcLabel IfcContext:ObjectType1->IfcLabel_5:IfcLabel0 IfcLabel_6 IfcLabel IfcContext:LongName1->IfcLabel_6:IfcLabel0 IfcDerivedUnit IfcDerivedUnit 1. Elements [1:?] DerivedUnitType 2. UnitType [1:1] 3. UserDefinedType [0:1] 4. Name [0:1] IfcUnitAssignment:Units1->IfcDerivedUnit:IfcDerivedUnit0 IfcMonetaryUnit IfcMonetaryUnit 1. Currency [1:1] IfcUnitAssignment:Units1->IfcMonetaryUnit:IfcMonetaryUnit0 IfcSIUnit IfcSIUnit 1. Dimensions [1:1] SIUnitType 2. UnitType [1:1] 3. Prefix [0:1] SIUnitName 4. Name [1:1] IfcUnitAssignment:Units1->IfcSIUnit:IfcSIUnit0 IfcConversionBasedUnit IfcConversionBasedUnit 1. Dimensions [1:1] ConversionUnitType 2. UnitType [1:1] ConversionUnitName 3. Name [1:1] 4. ConversionFactor [1:1]      HasExternalReference [0:?] IfcUnitAssignment:Units1->IfcConversionBasedUnit:IfcConversionBasedUnit0 IfcConversionBasedUnitWithOffset IfcConversionBasedUnitWithOffset 1. Dimensions [1:1] 2. UnitType [1:1] 3. Name [1:1] 4. ConversionFactor [1:1]      HasExternalReference [0:?] 5. ConversionOffset [1:1] IfcUnitAssignment:Units1->IfcConversionBasedUnitWithOffset:IfcConversionBasedUnitWithOffset0 IfcDerivedUnitEnum IfcDerivedUnitEnum IfcDerivedUnit:UnitType1->IfcDerivedUnitEnum:IfcDerivedUnitEnum0 IfcDerivedUnitElement IfcDerivedUnitElement 1. Unit [1:1] 2. Exponent [1:1] IfcDerivedUnit:Elements1->IfcDerivedUnitElement:IfcDerivedUnitElement0 IfcLabel_0 IfcLabel IfcDerivedUnit:UserDefinedType1->IfcLabel_0:IfcLabel0 IfcLabel_1 IfcLabel IfcMonetaryUnit:Currency1->IfcLabel_1:IfcLabel0 IfcUnitEnum_0 IfcUnitEnum IfcSIUnit:UnitType1->IfcUnitEnum_0:IfcUnitEnum0 IfcSIPrefix IfcSIPrefix IfcSIUnit:Prefix1->IfcSIPrefix:IfcSIPrefix0 IfcSIUnitName IfcSIUnitName IfcSIUnit:Name1->IfcSIUnitName:IfcSIUnitName0 IfcDimensionalExponents_0 IfcDimensionalExponents 1. LengthExponent [1:1] 2. MassExponent [1:1] 3. TimeExponent [1:1] 4. ElectricCurrentExponent [1:1] 5. ThermodynamicTemperatureExponent [1:1] 6. AmountOfSubstanceExponent [1:1] 7. LuminousIntensityExponent [1:1] IfcConversionBasedUnit:Dimensions1->IfcDimensionalExponents_0:IfcDimensionalExponents0 IfcUnitEnum_1 IfcUnitEnum IfcConversionBasedUnit:UnitType1->IfcUnitEnum_1:IfcUnitEnum0 IfcLabel_2 IfcLabel IfcConversionBasedUnit:Name1->IfcLabel_2:IfcLabel0 IfcMeasureWithUnit_0 IfcMeasureWithUnit 1. ValueComponent [1:1] 2. UnitComponent [1:1] IfcConversionBasedUnit:ConversionFactor1->IfcMeasureWithUnit_0:IfcMeasureWithUnit0 IfcDimensionalExponents_1 IfcDimensionalExponents 1. LengthExponent [1:1] 2. MassExponent [1:1] 3. TimeExponent [1:1] 4. ElectricCurrentExponent [1:1] 5. ThermodynamicTemperatureExponent [1:1] 6. AmountOfSubstanceExponent [1:1] 7. LuminousIntensityExponent [1:1] IfcConversionBasedUnitWithOffset:Dimensions1->IfcDimensionalExponents_1:IfcDimensionalExponents0 IfcUnitEnum_2 IfcUnitEnum IfcConversionBasedUnitWithOffset:UnitType1->IfcUnitEnum_2:IfcUnitEnum0 IfcLabel_3 IfcLabel IfcConversionBasedUnitWithOffset:Name1->IfcLabel_3:IfcLabel0 IfcMeasureWithUnit_1 IfcMeasureWithUnit 1. ValueComponent [1:1] 2. UnitComponent [1:1] IfcConversionBasedUnitWithOffset:ConversionFactor1->IfcMeasureWithUnit_1:IfcMeasureWithUnit0 IfcReal IfcReal IfcConversionBasedUnitWithOffset:ConversionOffset1->IfcReal:IfcReal0 IfcNamedUnit IfcNamedUnit 1. Dimensions [1:1] 2. UnitType [1:1] IfcDerivedUnitElement:Unit1->IfcNamedUnit:IfcNamedUnit0
Figure 4.1.9.9.A

General Usage

ApplicableEntity
IfcProject
Table 4.1.9.9.B

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