8.9 IfcMeasureResource

8.9.1 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;


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.

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 126 indicates measures, units, and corresponding data types.

Measure	Type	Unit	Symbol	Derivation	IfcSiUnitEnum	IfcUnitEnum	IfcDerivedUnitEnum	Defined value types
Absorbed dose, specific energy impact, kerma, absorbed dose index	SI / Derived	gray	Gy	J / kg	GRAY	ABSORBEDDOSEUNIT		IfcAbsorbedDoseMeasure
Acceleration	Derived			m / s2			ACCELERATIONUNIT	IfcAccelerationMeasure
Acidity (pH)	Derived	pH	pH	mol / l			PHUNIT	IfcPHMeasure
Activity (of radionuclide)	SI / Derived	becquerel	Bq	1 / s	BECQUEREL	RADIOACTIVITYUNIT		IfcRadioActivityMeasure
Amount of substance	SI/Basic	mole	mol		MOLE	AMOUNTOFSUBSTANCEUNIT		IfcAmountOfSubstanceMeasure
Angular velocity	Derived			rad / s			ANGULARVELOCITYUNIT	IfcAngularVelocityMeasure
Area	SI/Derived	square metre	m2	m2	SQUARE_METRE	AREAUNIT		IfcAreaMeasure
Area density	Derived			kg / m2			AREADENSITYUNIT	IfcAreaDensityMeasure
-								IfcBoolean
-								IfcComplexNumber
Compound plane angle	Compound			degree, min, s			COMPOUNDPLANEANGLEUNIT	IfcCompoundPlaneAngleMeasure
-								IfcContextDependentMeasure
-								IfcCountMeasure
-								IfcDescriptiveMeasure
Capacitance	SI / Derived	farad	F	C / V	FARAD	ELECTRICCAPACITANCEUNIT		IfcElectricCapacitanceMeasure
Celsius temperature	SI / Basic	degree Celsius	ºC	1 ºC = 1 K	DEGREE_CELSIUS	THERMODYNAMICTEMPERATUREUNIT		IfcThermodynamicTemperatureMeasure
Curvatue	Derived			rad / m			CURVATUREUNIT	IfcCurvatureMeasure
-								IfcDate (lexical representation according to ISO 8601)
-								IfcDateTime (lexical representation according to ISO 8601)
Dose equivalent, dose equivalent index	SI / Derived	sievert	Sv	J / kg	SIEVERT	DOSEEQUIVALENTUNIT		IfcDoseEquivalentMeasure
-								IfcDuration (lexical representation according to ISO 8601)
Dynamic viscosity	Derived			Pa · s			DYNAMICVISCOSITYUNIT	IfcDynamicViscosityMeasure
Electric charge, quantity of electricity	SI / Derived	coulomb	C	A · s	COULOMB	ELECTRICCHARGEUNIT		IfcElectricChargeMeasure
Electric conductance	SI / Derived	siemens	S	1 / W	SIEMENS	ELECTRICCONDUCTANCEUNIT		IfcElectricConductanceMeasure
Electric current	SI / Basic	ampere	A		AMPERE	ELECTRICCURRENTUNIT		IfcElectricCurrentMeasure
Electric potential, potential difference, tension, electromotive force	SI / Derived	volt	V	W / A	VOLT	ELECTRICVOLTAGEUNIT		IfcElectricVoltageMeasure
Electric resistance	SI / Derived	ohm	W	V / A	OHM	ELECTRICRESISTANCEUNIT		IfcElectricResistanceMeasure
Energy, work, quantity of heat	SI / Derived	joule	J	N · m	JOULE	ENERGYUNIT		IfcEnergyMeasure
Force	SI / Derived	newton	N	kg · m / s2	NEWTON	FORCEUNIT		IfcForceMeasure
Frequency	SI / Derived	hertz	Hz	1 / s	HERTZ	FREQUENCYUNIT		IfcFrequencyMeasure
Heat flux density	Derived			W / m2			HEATFLUXDENSITYUNIT	IfcHeatFluxDensityMeasure
Heating value	Derived			J / kg			HEATINGVALUEUNIT	IfcHeatingValueMeasure
-								IfcIdentifier
Illuminance	SI / Derived	lux	lx	lm / m2	LUX	ILLUMINANCEUNIT		IfcIlluminanceMeasure
Inductance	SI / Derived	henry	H	Wb / A	HENRY	INDUCTANCEUNIT		IfcInductanceMeasure
-								IfcInteger
(Integer) Count rate	Derived			1 / s			INTEGERCOUNTRATEUNIT	IfcIntegerCountRateMeasure
Ion concentration	Derived			g / l			IONCONCENTRATIONUNIT	IfcIonConcentrationMeasure
Isothermal moisture capacity	Derived			m3 / kg			ISOTHERMALMOISTURECAPACITYUNIT	IfcIsothermalMoistureCapacityMeasure
Kinematic viscosity	Derived			m2 / s			KINEMATICVISCOSITYUNIT	IfcKinematicViscosityMeasure
Length	SI / Basic	metre	m		METRE	LENGTHUNIT		IfcLengthMeasure
-								IfcLabel
Linear force	Derived			N / m			LINEARFORCEUNIT	IfcLinearForceMeasure
Linear moment	Derived			N · m / m			LINEARMOMENTUNIT	IfcLinearMomentMeasure
Linear stiffness	Derived			N / m			LINEARSTIFFNESSUNIT	IfcLinearStiffnessMeasure
Linear velocity	Derived			m / s			LINEARVELOCITYUNIT	IfcLinearVelocityMeasure
-								IfcLogical
Luminous flux	SI / Derived	lumen	lm	cd · sr	LUMEN	LUMINOUSFLUXUNIT		IfcLuminousFluxMeasure
Luminous intensity	SI / Basic	candela	cd		CANDELA	LUMINOUSINTENSITYUNIT		IfcLuminousIntensityMeasure
Luminous intensity distribution	Derived			cd / lm			LUMINOUSINTENSITYDISTRIBUTIONUNIT	IfcLuminousIntensityDistributionMeasure
Magnetic flux	SI / Derived	weber	Wb	V · s	WEBER	MAGNETICFLUXUNIT		IfcMagneticFluxMeasure
Magnetic flux density	SI / Derived	tesla	T	Wb / m2	TESLA	MAGNETICFLUXDENSITYUNIT		IfcMagneticFluxDensityMeasure
Mass	SI / Basic	gram	g (kg)		GRAM	MASSUNIT		IfcMassMeasure
Mass density	Derived			kg / m3			MASSDENSITYUNIT	IfcMassDensityMeasure
Mass flow rate	Derived			kg / s			MASSFLOWRATEUNIT	IfcMassFlowRateMeasure
Mass per length	Derived			kg / m			MASSPERLENGTHUNIT	IfcMassPerLengthMeasure
Modulus of elasticity	Derived			N / m2			MODULUSOFELASTICITYUNIT	IfcModulusOfElasticityMeasure
Modulus of linear subgrade reaction	Derived			N / m2			MODULUSOFLINEARSUBGRADEREACTIONUNIT	IfcModulusOfLinearSubgradeReactionMeasure
Modulus of rotational subgrade reaction	Derived			N · m / m · rad			MODULUSOFROTATIONALSUBGRADEREACTIONUNIT	IfcModulusOfRotationalSubgradeReactionMeasure
Modulus of subgrade reaction	Derived			N / m3			MODULUSOFSUBGRADEREACTIONUNIT	IfcModulusOfSubgradeReactionMeasure
Moisture diffusivity	Derived			m3 / s			MOISTUREDIFFUSIVITYUNIT	IfcMoistureDiffusivityMeasure
Molecular weight	Derived			g / mol			MOLECULARWEIGHTUNIT	IfcMolecularWeightMeasure
Moment of inertia	Derived			m4			MOMENTOFINERTIAUNIT	IfcMomentOfInertiaMeasure
-								IfcMonetaryMeasure
(Non negative length)			m			LENGTHUNIT		IfcNonNegativeLengthMeasure
-								IfcNormalisedRatioMeasure
-								IfcNumericMeasure
-								IfcParameterValue
Planar force	Derived			N / m2			PLANARFORCEUNIT	IfcPlanarForceMeasure
Plane angle	SI / Derived	radian	rad	m / m = 1	RADIAN	PLANEANGLEUNIT		IfcPlaneAngleMeasure
(Positive length)			m			LENGTHUNIT		IfcPositiveLengthMeasure
(Positive plane angle)			rad			PLANEANGLEUNIT		IfcPositivePlaneAngleMeasure
-								IfcPositiveRatioMeasure
Power	SI / Derived	watt	W	J / s	WATT	POWERUNIT		IfcPowerMeasure
Pressure, stress	SI / Derived	pascal	Pa	N / m2	PASCAL	PRESSUREUNIT		IfcPressureMeasure
-								IfcRatioMeasure
-								IfcReal
Rotational frequency	Derived			cycles / s			ROTATIONALFREQUENCYUNIT	IfcRotationalFrequencyMeasure
Rotational mass	Derived			kg · m2			ROTATIONALMASSUNIT	IfcRotationalMassMeasure
Rotational stiffness	Derived			N · m / rad			ROTATIONALSTIFFNESSUNIT	IfcRotationalStiffnessMeasure
Sectional area integral	Derived			m5			SECTIONALAREAINTEGRALUNIT	IfcSectionalAreaIntegralMeasure
Section modulus	Derived			m3			SECTIONMODULUSUNIT	IfcSectionModulusMeasure
Shear modulus	Derived			N / m2			SHEARMODULUSUNIT	IfcShearModulusMeasure
Solid angle	SI / Derived	steradin	sr	m2 / m2 = 1	STERADIAN	SOLIDANGLEUNIT		IfcSolidAngleMeasure
Sound power		watt	W	W			SOUNDPOWERUNIT	IfcSoundPowerMeasure
Sound power level		decibel	db	W / W			SOUNDPOWERLEVELUNIT	IfcSoundPowerLevelMeasure
Sound pressure		pascal	Pa	Pa			SOUNDPRESSUREUNIT	IfcSoundPressureMeasure
Sound pressure level		decibel	db	Pa / Pa			SOUNDPRESSURELEVELUNIT	IfcSoundPressureLevelMeasure
Specific heat capacity	Derived			J / kg· K			SPECIFICHEATCAPACITYUNIT	IfcSpecificHeatCapacityMeasure
Temperature gradient				K / m			TEMPERATUREGRADIENTUNIT	IfcTemperatureGradientMeasure
Temperature change				K / s			TEMPERATURERATEOFCHANGEUNIT	IfcTemperatureRateOfChangeMeasure
-								IfcText
Thermal admittance	Derived			W / m2 · K			THERMALADMITTANCEUNIT	IfcThermalAdmittanceMeasure
Thermal conductivity	Derived			W / m · K			THERMALCONDUCTIVITYUNIT	IfcThermalConductivityMeasure
Thermal expansion coefficient				1 / K			THERMALEXPANSIONCOEFFICIENTUNIT	IfcThermalExpansionCoefficientMeasure
Thermal resistance	Derived			m2 · K / W			THERMALRESISTANCEUNIT	IfcThermalResistanceMeasure
Thermal transmittance	Derived			W / m2 · K			THERMALTRANSMITTANCEUNIT	IfcThermalTransmittanceMeasure
Thermodynamic temperature	SI / Basic	kelvin	K		KELVIN	THERMODYNAMICTEMPERATUREUNIT		IfcThermodynamicTemperatureMeasure
-								IfcTime (lexical representation according to ISO 8601)
Time	SI / Basic	second	s		SECOND	TIMEUNIT		IfcTimeMeasure
-								IfcTimeStamp
Torque	Derived			N· m			TORQUEUNIT	IfcTorqueMeasure
Vapor permeability	Derived			kg / s · m · Pa			VAPORPERMEABILITYUNIT	IfcVaporPermeabilityMeasure
Volume	SI / Derived	cubic metre	m3	m3	CUBIC_METRE	VOLUMEUNIT		IfcVolumeMeasure
Volumetric flow rate	Derived			m3 / s			VOLUMETRICFLOWRATEUNIT	IfcVolumetricFlowRateMeasure
Warping constant				m6			WARPINGCONSTANTUNIT	IfcWarpingConstantMeasure
Warping moment	Derived			N · m2			WARPINGMOMENTUNIT	IfcWarpingMomentMeasure

Table 126 — Measures and units