IFC 4.3.x development

8.18.3.9 IfcProjectedCRS

Change log

IFC4

added

8.18.3.9.1 Semantic definitions at the entity

IfcProjectedCRS is a coordinate reference system of the map to which the map translation of the local engineering coordinate system of the construction or facility engineering project relates. The MapProjection and MapZone attributes uniquely identify the projection to the underlying geographic coordinate reference system, provided that they are well-known in the receiving application. The projected coordinate reference system is assumed to be a 2D or 3D right-handed Cartesian coordinate system, the optional MapUnit attribute can be used determine the length unit used by the map.

NOTE  Definition from OpenGIS Abstract Specification, Topic 2:
A 2D (or with vertical coordinate axis 3D) coordinate reference system used to approximate the shape of the earth on a planar surface, but in such a way that the distortion that is inherent to the approximation is carefully controlled and known. Distortion correction is commonly applied to calculated bearings and distances to produce values that are a close match to actual field values.

The unambiguous identifier by which the coordinate reference system is know, is stored in the inherited Name attribute. Well defined identifiers include the map projection and also the map zone information. In these cases the MapProjection and the MapZone attributes can be omitted.

EXAMPLE  The identifier 'EPSG:25832' defines the map projection 'UTM' and the zone '32N' in additon to the geodetic and vertical datum.

HISTORY  New entity in IFC4.

8.18.3.9.1 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.
IfcProjectedCRS
5 MapProjection OPTIONAL IfcIdentifier Name by which the map projection is identified.

> EXAMPLE  map projects include:

> * UTM

> * Gaus-Krueger
6 MapZone OPTIONAL IfcIdentifier Name by which the map zone, relating to the MapProjection, is identified.

> EXAMPLE 

> * for UTM, the zone number, like 32 for UTM32

> * for Gaus-Krueger, the zones of longitudinal width, like 3'
7 MapUnit OPTIONAL IfcNamedUnit Unit of the coordinate axes composing the map coordinate system.

> NOTE 1  Only length measures are in scope and all two or three axes of the map coordinate system shall have the same length unit.

> NOTE 2  If MapUnit is omited, the unit for the coordinate axes is taken from the default units, as stated in IfcProject.UnitInContext.

8.18.3.9.2 Entity inheritance

dot_inheritance IfcProjectedCRS IfcProjectedCRS IfcCoordinateReferenceSystem IfcCoordinateReferenceSystem IfcProjectedCRS->IfcCoordinateReferenceSystem

8.18.3.9.3 Formal representations

ENTITY IfcProjectedCRS
 SUBTYPE OF (IfcCoordinateReferenceSystem);
	MapProjection : OPTIONAL IfcIdentifier;
	MapZone : OPTIONAL IfcIdentifier;
	MapUnit : OPTIONAL IfcNamedUnit;
 WHERE
	IsLengthUnit : NOT(EXISTS(MapUnit)) OR (MapUnit.UnitType = IfcUnitEnum.LENGTHUNIT);
END_ENTITY;