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IfcVirtualGridIntersection

Definition from IAI: The virtual grid intersection (IfcVirtualGridIntersection) defines the derived location of the intersection between two grid axes. Offset values may be given to set an offset distance to the grid axis for the calculation of the virtual grid intersection.

The two intersecting axes (IntersectingAxes) define the intersection point, which exact location (in terms of the Cartesian point representing the intersection) has to be calculated from the geometric representation of the two participating curves.

Offset values may be given (OffsetDistances). If given, the position within the list of OffsetDistances corresponds with the position within the list of IntersectingAxes. Therefore:

Illustration

2D offsets Two offset distances given, the virtual intersection is defined in the xy plane of the grid axis placement.
3D offsets Three offset distances given, the virtual intersection is defined by an offset (in direction of the z-axis of the design grid placement) to the virtual intersection in the xy plane of the grid axis placement.

The distance of the offset curve (OffsetDistances[n]) is measured from the basis curve. The distance may be positive, negative or zero. A positive value of distance defines an offset in the direction which is normal to the curve in the sense of an anti-clockwise rotation through 90 degrees from the tangent vector T at the given point. (This is in the direction of orthogonal complement(T).) This can be reverted by the SameSense attribute at IfcGridAxis which may switch the sense of the AxisCurve.

Illustration

offset direction example of a negative offset
  • IntersectingAxes[1].AxisCurve is an IfcTrimmedCurve with an IfcCircle as BasisCurve and SenseAgreement = TRUE.
  • IntersectingAxes[1].SameSense = TRUE.
  • OffsetDistances[1] is a negative length measure
the figure shows the side of the offset.
HISTORY: New entity in IFC Release 1.5. The definition has changed after version 2x beta 2.
ISSUES: See issue log for changes made in IFC Release 2x. The entity name was changed from IfcConstraintRelIntersection.

EXPRESS specification:

ENTITY IfcVirtualGridIntersection;
IntersectingAxes  :  LIST [2:2] OF UNIQUE IfcGridAxis;
OffsetDistances  :  LIST [2:3] OF IfcLengthMeasure;
END_ENTITY;

Attribute definitions:

IntersectingAxes  :  Two grid axes which intersects at exactly one intersection (see also informal proposition at IfcGrid). If attribute OffsetDistances is omited, the intersection defines the placement or ref direction of a grid placement directly. If OffsetDistances are given, the intersection is defined by the offset curves to the grid axes.
OffsetDistances  :  Offset distances to the grid axes. If given, it defines virtual offset curves to the grid axes. The intersection of the offset curves specify the virtual grid intersection.

References (2):

Name Type Referred through Express-G
IfcGridPlacement Entity
Attribute 'PlacementLocation'
Attribute 'PlacementRefDirection'
Diagram 1

Inheritance graph

ENTITY IfcVirtualGridIntersection;
ENTITY IfcVirtualGridIntersection;
IntersectingAxes  :  LIST [2:2] OF UNIQUE IfcGridAxis;
OffsetDistances  :  LIST [2:3] OF IfcLengthMeasure;
END_ENTITY;