8.18.3.3 IfcGeographicCRS

8.18.3.3.1 Semantic definition

A IfcGeographicCRS is a coordinate reference system (CRS) that uses a three-dimensional ellipsoid surface to reference locations on the Earth. Any location on Earth can be described by a point with longitude and latitude coordinates and the height above or below the ellipsoid surface.

8.18.3.3.2 Entity inheritance

8.18.3.3.3 Attributes

#	Attribute	Type	Description
IfcCoordinateReferenceSystem (5)
1	Name	OPTIONAL IfcLabel	Name by which the coordinate reference system is identified. NOTE The name shall be taken from the list recognized by the European Petroleum Survey Group EPSG. It should then be qualified by the EPSG namespace, for example as 'EPSG:5555'. NOTE The Name shall contain only one EPSG code. When there is not one EPSG that unambiguously identifies the CRS, IfcWellKnownText shall be used. Combining multiple EPSG codes in one string for Name (e.g., 'EPSG:2056,EPSG:5728') is not allowed. NOTE The name shall be 'WKT' if an EPSG code does not exist for the coordinate reference system (CRS). In this case, the CRS shall be further specified using the IfcWellKnownText entity.
2	Description	OPTIONAL IfcText	Informal description of this coordinate reference system.
3	GeodeticDatum	OPTIONAL IfcIdentifier	Name by which this datum is identified. The geodetic datum is associated with the coordinate reference system and indicates the shape and size of the rotation ellipsoid and this ellipsoid's connection and orientation to the actual globe/earth. It needs to be provided, if the Name identifier does not unambiguously define the geodetic datum as well. EXAMPLE geodetic datums include:
	HasCoordinateOperation	SET [0:1] OF IfcCoordinateOperation FOR SourceCRS	Indicates conversion between coordinate reference systems. In particular it refers to an IfcCoordinateOperation between this coordinate reference system, and another coordinate reference system.
	WellKnownText	SET [0:1] OF IfcWellKnownText FOR CoordinateReferenceSystem	Well Known Text (WKT) definition for this coordinate reference system inversely associated as a IfcWellKnownText entity.
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IfcGeographicCRS (3)
4	PrimeMeridian	OPTIONAL IfcIdentifier	The identification of the meridian defining zero longitude in the used geographic CRS.
5	AngleUnit	OPTIONAL IfcNamedUnit	Unit of latitude and longitude coordinate axes composing the geographic coordinate system. NOTE Only plane angle measures are in scope and both longitude and latitude coordinate axes of the geographic coordinate system shall have the same plane angle unit. NOTE If AngleUnit is omitted, the unit for latitude and longitude coordinate axes is taken from the default project angle units, as stated in IfcProject.UnitInContext.
6	HeightUnit	OPTIONAL IfcNamedUnit	Unit of the height coordinate axis of the geographic coordinate system. NOTE Only length measures are in scope. NOTE If HeightUnit is omitted, the unit for the height coordinate axis is taken from the default project length units, as stated in IfcProject.UnitInContext.

Table 8.18.3.3.B

8.18.3.3.4 Formal propositions

Name	Description
AngleUnitIsPlaneAngle	The type of AngleUnit in the operation shall be IfcUnitEnum.PLANEANGLEUNIT.
`NOT(EXISTS(SELF.AngleUnit)) OR (SELF.AngleUnit.UnitType=IfcUnitEnum.PLANEANGLEUNIT)`
HeightUnitIsLength	The type of HeightUnit in the operation shall be IfcUnitEnum.LENGTHUNIT.
`NOT(EXISTS(SELF.HeightUnit)) OR (SELF.HeightUnit.UnitType=IfcUnitEnum.LENGTHUNIT)`

Table 8.18.3.3.C

8.18.3.3.5 Examples

Figure 8.18.3.3.A

Geographic Referencing Rigid Operation

8.18.3.3.6 Formal representation

ENTITY IfcGeographicCRS
 SUBTYPE OF (IfcCoordinateReferenceSystem);
	PrimeMeridian : OPTIONAL IfcIdentifier;
	AngleUnit : OPTIONAL IfcNamedUnit;
	HeightUnit : OPTIONAL IfcNamedUnit;
 WHERE
	AngleUnitIsPlaneAngle : NOT(EXISTS(SELF.AngleUnit)) OR (SELF.AngleUnit.UnitType=IfcUnitEnum.PLANEANGLEUNIT);
	HeightUnitIsLength : NOT(EXISTS(SELF.HeightUnit)) OR (SELF.HeightUnit.UnitType=IfcUnitEnum.LENGTHUNIT);
END_ENTITY;

8.18.3.3.7 References

IfcCoordinateReferenceSystem