# 8.7.3.12 IfcGridPlacement

## 8.7.3.12.1 Semantic definition

IfcGridPlacement provides a specialization of IfcObjectPlacement in which the placement and axis direction of the object coordinate system is defined by a reference to the design grid as defined in IfcGrid.

The location of the object coordinate system is given by the attribute PlacementLocation. It is defined as an IfcVirtualGridIntersection, that is, an intersection between two grid axes with optional offsets.

The axis direction of the x-axis of the object coordinate system is given either:

• PlacementRefDirection = NIL: by the tangent of the first grid axis (PlacementLocation.IntersectingAxes[1]) at the virtual intersection (maybe using the offset curve, if PlacementLocation.OffsetDistances is given);
• PlacementRefDirection = IfcDirection: by the explicitly provided direction information;
• PlacementRefDirection = IfcVirtualGridIntersection: by the tangent between the virtual grid intersection of PlacementLocation and the virtual grid intersection of PlacementRefDirection. Offsets as potentially provided in the IfcVirtualGridIntersection's of PlacementLocation and PlacementRefDirection have to be taken into account.

The direction of the y-axis of the IfcGridPlacement is the orthogonal complement to the x-axis. The plane defined by the x and y axis shall be co-planar to the xy plane of the local placement of the IfcGrid.

The direction of the z-axis is the orientation of the cross product of the x-axis and the y-axis, i.e. the z-axis of the IfcGridPlacement shall be co-linear to the z-axis of the local placement of the IfcGrid.

The following figures/ show the usage of placement location and direction for an IfcGridPlacement.

Figure 8.7.3.12.A illustrates the case where PlacementRefDirection is not given - the object coordinate system is defined by:

• its location: given by the virtual grid intersection of PlacementLocation
• its x-axis direction: given by the tangent of the first intersecting axis in the offset location of the virtual grid intersection

Figure 8.7.3.12.B illustrates the case where PlacementRefDirection is given as an IfcDirection- the object coordinate system is defined by:

• its location: given by the virtual grid intersection of PlacementLocation
• its x-axis direction: given by the DirectionRatios of the IfcDirection, only the ratios for x and y are taken into account,

Figure 8.7.3.12.C illustrates the case where PlacementRefDirection is given as an IfcVirtualGridIntersection- the object coordinate system is defined by:

• its location: given by the virtual grid intersection of PlacementLocation
• its x-axis direction: given by the tangent of the line between the virtual grid intersection of the PlacementLocation and the virtual grid intersection of the PlacementRefDirection.

## 8.7.3.12.4 Formal representation

ENTITY IfcGridPlacement
SUBTYPE OF (IfcObjectPlacement);
PlacementLocation : IfcVirtualGridIntersection;
PlacementRefDirection : OPTIONAL IfcGridPlacementDirectionSelect;
END_ENTITY;