IfcDerivedUnit

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.

HISTORY This entity has changes after IFC Release 1.5.1.
This entity has changes after IFC Release 2.0.
ISSUE See issue and change log for issues and changes made in IFC Release 2x.

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;

Attribute definitions:

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.

Formal Propositions:

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.