#### 8.16.3.1 IfcGeometricRepresentationContext

| Geometrische Repräsentation - Kontext

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 134.

> NOTE Definition according to ISO/CD 10303-42:1992
> A geometric representation context is a representation context in which the geometric representation items are geometrically founded.

| 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.
Figure 134 — Definition of the true north direction

> NOTE The inherited attribute ContextType shall have one of the following recognized values: '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. See Figure 135 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.
Figure 135 — Example of using geometric representation contexts

> NOTE Entity adapted from geometric_representation_context defined in ISO 10303-42.

> HISTORY New entity in IFC2.0.

> IFC2x3 CHANGE Applicable values for ContextType are only 'Model', 'Plan', and 'NotDefined'. All other contexts are now handled by IfcGeometricRepresentationSubContext.

XSD Specification:

<xs:element name="IfcGeometricRepresentationContext" type="ifc:IfcGeometricRepresentationContext" substitutionGroup="ifc:IfcRepresentationContext" nillable="true"/>
<xs:complexType name="IfcGeometricRepresentationContext">
<xs:complexContent>
<xs:extension base="ifc:IfcRepresentationContext">
<xs:sequence>
<xs:element name="WorldCoordinateSystem" minOccurs="0">
<xs:complexType>
<xs:group ref="ifc:IfcAxis2Placement"/>
</xs:complexType>
</xs:element>
<xs:element name="TrueNorth" type="ifc:IfcDirection" nillable="true" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="CoordinateSpaceDimension" type="ifc:IfcDimensionCount" use="optional"/>
</xs:extension>
</xs:complexContent>
</xs:complexType>

EXPRESS Specification:

ENTITY IfcGeometricRepresentationContext

| SUBTYPE OF (IfcRepresentationContext);

CoordinateSpaceDimension | : | IfcDimensionCount;
Precision | : | OPTIONAL IfcStrippedOptional;
WorldCoordinateSystem | : | IfcAxis2Placement;
TrueNorth | : | OPTIONAL IfcDirection;

| INVERSE

| WHERE

WR11 | : | NOT(EXISTS(TrueNorth)) OR (HIINDEX(TrueNorth.DirectionRatios) = 2);

END_ENTITY;

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 present, it defaults to 0. 1., meaning that the positive Y axis of the project coordinate system equals the geographic northing direction.

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 IfcStrippedOptional;
WorldCoordinateSystem | : | IfcAxis2Placement;
TrueNorth | : | OPTIONAL IfcDirection;

| INVERSE

END_ENTITY;

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