8.19.2.3 IfcGeometricRepresentationContext
Definition from ISO/CD 1030342:1992: 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.

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 xyplane 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.

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
1030342:1994 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


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

EXPRESSG Specification
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 1E5 to 1E8 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 xyplane 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 twodimensional direction.

Inheritance Graph
ENTITY IfcGeometricRepresentationContext
