IFC 4.3.0.1 (IFC4X3_TC1)

# 8.18.3.3 IfcGeometricRepresentationContext

## 8.18.3.3.1 Semantic definition

The IfcGeometricRepresentationContext defines the context that applies to several shape representations of products within a project. It defines the type of the context in which the shape representation is defined, and the numeric precision applicable to the geometric representation items defined in this context. In addition it can be used to offset the project coordinate system from a global point of origin, using the WorldCoordinateSystem attribute. The main representation context may also provide the true north direction, see Figure 8.18.3.3.A.

The TrueNorth attribute should be provided, if the y axis of the WorldCoordinateSystem does not point to the global northing. Direction of the true north, or geographic northing direction, relative to the underlying project coordinate system as established by the attribute WorldCoordinateSystem. It is given by a 2 dimensional direction within the xy-plane of the project coordinate system. If not resent, it defaults to [0.,1.] - i.e. the positive Y axis of the project coordinate system equals the geographic northing direction. The direction is provided within project coordinate system and identifies the true north direction.

The "true North as seen in CAD" within Figure 8.18.3.3.A shows the correct interpretation of the TrueNorth direction.

If a conversion to a geographic coordinate system is included by virtue of the HasCoordinateOperation attribute, then the TrueNorth attribute shall be omitted or included only for informational purposes. It shall not be added to any transformation already applied by an IfcCoordinateOperation.

The use of one instance of IfcGeometricRepresentationContext to represent the model (3D) view is mandatory, the use of a second instance of IfcGeometricRepresentationContext to represent the plan (2D) view is optional (but needs to be given, if there are scale dependent plan views), the additional scale or view dependent contexts need to be handled by using the subtype IfcGeometricRepresentationSubContext pointing to the model view (or the plan view) as the ParentContext. See Figure 8.18.3.3.B for an example using geometric representation contexts for 3D and 2D with assigned sub contexts.

Use of representation contexts defined at IfcProject for 3D model and 2D plan context, including sub context definitions for different target scales. There shall always be a maximum of one geometric representation context for 2D and for 3D coordinate space.

## 8.18.3.3.6 Formal representation

ENTITY IfcGeometricRepresentationContext
SUPERTYPE OF (ONEOF
(IfcGeometricRepresentationSubContext))
SUBTYPE OF (IfcRepresentationContext);
CoordinateSpaceDimension : IfcDimensionCount;
Precision : OPTIONAL IfcReal;
WorldCoordinateSystem : IfcAxis2Placement;
TrueNorth : OPTIONAL IfcDirection;
INVERSE
HasSubContexts : SET [0:?] OF IfcGeometricRepresentationSubContext FOR ParentContext;
HasCoordinateOperation : SET [0:1] OF IfcCoordinateOperation FOR SourceCRS;
WHERE
North2D : NOT(EXISTS(TrueNorth)) OR (HIINDEX(TrueNorth.DirectionRatios) = 2);
END_ENTITY;

## 8.18.3.3.7 References

### 8.18.3.3.8 Changelog

#### 8.18.3.3.8.1 IFC4

• inverse attribute, HasCoordinateOperation
• where rule, North2D
• attribute Precision type, Changed from "real" to "IfcReal"