8.11 IfcMeasureResource

8.11.1 Schema Definition

The IfcMeasureResource schema specifies units and defined measure types that may be assigned to quantities.

NOTE  The fundamental unit types used in this schema are based on the SI system defined in ISO 1000+A1, 1992, 1998. Units in measurement systems other than SI may be derived using this schema. Many definitions declared in the IfcMeasureResource schema is adapted from ISO 10303-41

NOTE  In the definitions of the unit exponents the use of superscript font has been omitted. Therefore, m2 means square metre, m3 means cubic meter.

In different types of units there are five basic cases:

• Basic SI-units, which cover a number of fundamental units of mainly physical quantities defined by ISO-1000 such as meter or millimeter as unit for length measure or square meter as a unit for area measure. The unit may have a scaling prefix (for example: milli, kilo).
• Derived SI-units, which are defined as a derivation of the basic SI-units, for example, Newton (kg m / s2) as a unit of force. Both basic and derived SI-units are in the IFCs represented by IfcSIUnit.
• Conversion based units, which can be derived from SI-units by a scaling factor; e.g. inch which can be defined using SI-unit for length measure, i.e. an inch is 25.4 millimeters.
• Derived units, which can be defined as a derivation or combination of a number of basic units. In a derived unit each of the basic unit "component" has a dimensional exponent in defining the derived unit; e.g. kg / m2, where kilogram (kg) has dimensional exponent 1 and meter (m) has exponent -2.
• Context dependent units, which cannot be defined as conversion based unit using SI-units.

With regard to the usage of the measure defined types (for example, IfcLengthMeasure, IfcTimeMeasure) as attribute datatypes in this specification, there are three basic cases:

1. The datatype of an attribute of an entity type is a measure defined type as such without possibility on an instance level to define the unit of the measure value:

   ENTITY IfcBoundingBox
SUBTYPE OF(IfcGeometricRepresentationItem);
Corner : IfcCartesianPoint;
XDim : IfcPositiveLengthMeasure;
YDim : IfcPositiveLengthMeasure;
ZDim : IfcPositiveLengthMeasure;
END_ENTITY;


   In this case, it is the global unit assignment for the corresponding unit for the measure type that defines the unit for all the usages of this defined measure type (except for cases 2 and 3 below).
2. The datatype of an attribute is IfcMeasureWithUnit, which allows for definition of unit per instance of that entity type, independent and possibly overriding the global unit assignment:

   ENTITY IfcConversionBasedUnit
SUPERTYPE OF (ONEOF
(IfcConversionBasedUnitWithOffset))
SUBTYPE OF (IfcNamedUnit);
Name : IfcLabel;
ConversionFactor : IfcMeasureWithUnit;
END_ENTITY;


   In this case the relevant measure defined type (from the IfcMeasureWithUnit.ValueComponent : IfcValue select list) is not exactly defined by the schema, but implied by the context.
3. The entity type has a separate "unit" attribute which allows for defining the unit for another attribute of the entity type for representing the actual value:

   ENTITY IfcPropertySingleValue
SUBTYPE OF (IfcSimpleProperty);
NominalValue : IfcValue;
Unit : OPTIONAL IfcUnit;
END_ENTITY;


NOTE  In the example instantiations in the form of IFC data exchange files, mainly the measure and unit -relevant attributes are given the values; the other attributes are given no values (in the form of $-sign) independent of whether they should actually have values because of being nonoptional attributes.

Table 695 indicates measures, units, and corresponding data types.

Table 695 — Measures and units

 EXPRESS-G diagram

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