# 8.11 IfcMeasureResource

## 8.11.1 Schema definition

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

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{ .int-ref} 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:

First, 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).

Second, 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.

Third, 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;


Although in the cases 2 and 3 different units could be used for different instances of the same entity type or for the same measure type in attributes of different entity types, it is recommended not to mix different units for same measure defined types, if it can be avoided. Below some examples of each of the above basic cases are given.

Table 8.11.A indicates measures, units, and corresponding data types.

## 8.11.4 Property Sets

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## 8.11.5 Quantity Sets

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## 8.11.7 Rules

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## 8.11.8 PropertyEnumerations

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