8.19.2.3 IfcGeometricRepresentationContext

Definition from ISO/CD 10303-42:1992: A geometric representation context is a representation context in which the geometric representation items are geometrically founded. A geometric representation context is a distinct coordinate space, spatially unrelated to other coordinate spaces.

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.

TrueNorth
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 (see figure).

NOTE The inherited attribute ContextType shall have one of the following recognized values: 'Sketch', 'Outline', 'Design', 'Detail', 'Model', 'Plan', 'NotDefined'.

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.

representation context Use of representation contexts defined at IfcProject for 3D model and 2D plan context, including sub context definitions for different target scales.
NOTE The definition of this class relates to the STEP entity geometric_representation_context. Please refer to ISO/IS 10303-42:1994 for the final definition of the formal standard.
HISTORY New Entity in IFC Release 2.0
IFC2x Edition 3 CHANGE Applicable values for ContextType are only 'Model', 'Plan', and 'NotDefined'. All other sub contexts are now handled by the new subtype in IFC2x Edition 2 IfcGeometricRepresentationSubContext. Upward compatibility for file based exchange is guaranteed.

EXPRESS Specification

ENTITY IfcGeometricRepresentationContext
SUPERTYPE OF(IfcGeometricRepresentationSubContext)
SUBTYPE OF IfcRepresentationContext;
CoordinateSpaceDimension : IfcDimensionCount;
Precision : OPTIONAL REAL;
WorldCoordinateSystem : IfcAxis2Placement;
TrueNorth : OPTIONAL IfcDirection;
INVERSE
HasSubContexts : SET OF IfcGeometricRepresentationSubContext FOR ParentContext;
WHERE
WR11 : NOT(EXISTS(TrueNorth)) OR (HIINDEX(TrueNorth.DirectionRatios) = 2)
END_ENTITY;

EXPRESS-G Specification

Link to EXPRESS-G diagram

Attribute Definitions

CoordinateSpaceDimension : The integer dimension count of the coordinate space modeled in a geometric representation context.
Precision : Value of the model precision for geometric models. It is a double value (REAL), typically in 1E-5 to 1E-8 range, that indicates the tolerance under which two given points are still assumed to be identical. The value can be used e.g. to sets the maximum distance from an edge curve to the underlying face surface in brep models.
WorldCoordinateSystem : Establishment of the engineering coordinate system (often referred to as the world coordinate system in CAD) for all representation contexts used by the project.
Note  it can be used to provide better numeric stability if the placement of the building(s) is far away from the origin. In most cases however it would be set to origin: (0.,0.,0.) and directions x(1.,0.,0.), y(0.,1.,0.), z(0.,0.,1.).
TrueNorth : Direction of the true north, or geographic northing direction, relative to the underlying project coordinate system. 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.
HasSubContexts : The set of IfcGeometricRepresentationSubContexts that refer to this IfcGeometricRepresentationContext.
IFC2x Edition 3 CHANGE  New inverse attribute.

Formal Propositions

WR11 : The TrueNorth direction, if provided, shall be a two-dimensional direction.

Inheritance Graph

ENTITY IfcGeometricRepresentationContext
ENTITY IfcRepresentationContext
ContextIdentifier : OPTIONAL IfcLabel;
ContextType : OPTIONAL IfcLabel;
INVERSE
RepresentationsInContext : SET OF IfcRepresentation FOR ContextOfItems;
ENTITY IfcGeometricRepresentationContext
CoordinateSpaceDimension : IfcDimensionCount;
Precision : OPTIONAL REAL;
WorldCoordinateSystem : IfcAxis2Placement;
TrueNorth : OPTIONAL IfcDirection;
INVERSE
HasSubContexts : SET OF IfcGeometricRepresentationSubContext FOR ParentContext;
END_ENTITY;