IfcDerivedUnit

Definition from ISO/CD 10303-41:1992: A derived unit is an expression of units.

EXAMPLE: Newton per square millimetre is a derived unit.

NOTE Corresponding STEP name: derived_unit, please refer to ISO/IS 10303-41:1994 for the final definition of the formal standard.

HISTORY New entity in IFC Release 1.5.1.

EXPRESS specification:

ENTITY IfcDerivedUnit;

Elements

SET [1:?] OF IfcDerivedUnitElement;

UnitType

IfcDerivedUnitEnum;

UserDefinedType

OPTIONAL IfcLabel;

DERIVE

Dimensions

IfcDimensionalExponents := IfcDeriveDimensionalExponents(SELF);

WHERE

WR1	:	(SIZEOF (Elements) > 1) OR ((SIZEOF (Elements) = 1) AND (Elements[1].Exponent <> 1 ));
WR2	:	(UnitType <> IfcDerivedUnitEnum.USERDEFINED) OR ((UnitType = IfcDerivedUnitEnum.USERDEFINED) AND (EXISTS(SELF.UserDefinedType)));

END_ENTITY;

Elements	:	The group of units and their exponents that define the derived unit.
UnitType	:	Name of the derived unit chosen from an enumeration of derived unit types for use in IFC models.
Dimensions	:	Dimensional exponents derived using the function IfcDerivedDimensionalExponents using (SELF) as the input value.

WR1	:	Units as such shall not be re-defined as derived units.
WR2	:	When attribute UnitType has enumeration value USERDEFINED then attribute UserDefinedType shall also have a value.