IFC 4.3.x development

8.18.3.2 IfcCoordinateReferenceSystem

Change log

IFC4

added

8.18.3.2.1 Semantic definitions at the entity

The IfcCoordinateReferenceSystem is a definition of a coordinate reference system by means of qualified identifiers only. The interpretation of the identifier is expected to be well-known to the receiving software.

NOTE  Definition from OpenGIS Abstract Specification, Topic 2:
A coordinate reference system is a coordinate system which is related to the real world by a datum. The coordinate system is composed of a set of coordinate axes with specified units of measure. The datum specifies the relationship of a coordinate system to the earth. The resulting combination of coordinate system and datum is a coordinate reference system.

The unambiguous identifier by which the coordinate reference system is know, is stored in the Name attribute. Well defined identifiers include the geodetic and often also the vertical datum. In these cases the GeodeticDatum and the VerticalDatum can be omitted.

EXAMPLE  The identifier 'EPSG:25832' defines the geodetic datum 'ETRS89' in additon to the projection and the zone. 'EPSG:5555' defined the geodetic datum 'ETRS89' and the vertical datum 'DHHN92'.

NOTE  One widely-used, publicly-available authority is the European Petroleum Survey Group (EPSG), and use of this authority is currently specified in several OGC Implementation Specifications. Software used to transport IFC engineering models into GIS applications (and vice versa) is expected to have knowledge about the OGC Implementation Specifications.

HISTORY  New entity in IFC4.

8.18.3.2.1 Attributes

# Attribute Type Description
IfcCoordinateReferenceSystem
1 Name IfcLabel Name by which the coordinate reference system is identified.

> NOTE 1  The name shall be taken from the list recognized by the European Petroleum Survey Group EPSG. It should then be qualified by the EPSG name space, for example as 'EPSG:5555'.

> NOTE 2  The name shall be 'WKT' if an EPSG code does not exist for the CRS.
2 Description OPTIONAL IfcText Informal description of this coordinate reference system

> NOTE  In case Name equals 'WKT' the Description is well-known text that corresponds to ISO 19162 definition specifying the necessary parameters for the coordinate reference system.

Example for ellipsoid:

ELLIPSOID[<name>,<major axis>,<inverse flattening>,<length unit>]
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:

> * ED50

> * EUREF89

> * WSG84
4 VerticalDatum OPTIONAL IfcIdentifier Name by which the vertical datum is identified. The vertical datum is associated with the height axis of the coordinate reference system and indicates the reference plane and fundamental point defining the origin of a height system. It needs to be provided, if the Name identifier does not unambiguously define the vertical datum as well and if the coordinate reference system is a 3D reference system.

> EXAMPLE  vertical datums include:

> * DHHN92: the German 'Haupthöhennetz'

> * EVRS2007; the European Vertical Reference System
HasCoordinateOperationSET [0:1] OF IfcCoordinateOperation FOR SourceCRSIndicates conversion between coordinate systems. In particular it refers to an IfcCoordinateOperation between this coordinate reference system, and another Geographic coordinate reference system.

8.18.3.2.2 Entity inheritance

dot_inheritance IfcProjectedCRS IfcProjectedCRS IfcCoordinateReferenceSystem IfcCoordinateReferenceSystem IfcProjectedCRS->IfcCoordinateReferenceSystem

8.18.3.2.3 Formal representations

ENTITY IfcCoordinateReferenceSystem
 ABSTRACT SUPERTYPE OF (ONEOF
	(IfcProjectedCRS));
	Name : IfcLabel;
	Description : OPTIONAL IfcText;
	GeodeticDatum : OPTIONAL IfcIdentifier;
	VerticalDatum : OPTIONAL IfcIdentifier;
 INVERSE
	HasCoordinateOperation : SET [0:1] OF IfcCoordinateOperation FOR SourceCRS;
END_ENTITY;