IfcAxis2Placement2D

IfcAxis2Placement2D提供在二维空间中放置项目的位置和方向。属性RefDirection定义x轴，导出y轴。如果未给定属性RefDirection，则放置默认为P[1]（x轴）为[1.，0.]，P[2]（y轴）为[0,1]。

图说明了二维坐标系中IfcAxis2Placement2D的定义。

注：定义根据ISO/CD 10303-42:1992

两个相互垂直的轴在二维空间中的位置和方向。axis2_placement_2d是根据一个点（继承自placement supertype）和轴定义的。它可以用于在二维空间中定位和创建对象，并定义放置坐标系。图元包括形成放置坐标系原点的点。需要一个方向向量来完成放置坐标系的定义。参考方向定义放置X轴方向，放置Y轴由此衍生。

注：实体改编自ISO 10303-42中定义的axis2_placement_2d。

在IFC1.5中增加的新实体。

#	Attribute	Type	Cardinality	Description	C
IfcRepresentationItem
	LayerAssignment	IfcPresentationLayerAssignment @AssignedItems	S[0:1]	Assignment of the representation item to a single or multiple layer(s). The LayerAssignments can override a LayerAssignments of the IfcRepresentation it is used within the list of Items.	X
	StyledByItem	IfcStyledItem @Item	S[0:1]	Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve.	X
IfcGeometricRepresentationItem
IfcPlacement
1	Location	IfcCartesianPoint	[1:1]	The geometric position of a reference point, such as the center of a circle, of the item to be located.	X
	Dim :=Location.Dim	IfcDimensionCount	[1:1]	The space dimensionality of this class, derived from the dimensionality of the location.	X
IfcAxis2Placement2D
2	RefDirection	IfcDirection	[0:1]	The direction used to determine the direction of the local X axis. If a value is omited that it defaults to [1.0, 0.0.].	X
	P :=IfcBuild2Axes(RefDirection)	IfcDirection	L[2:2]	P[1]: The normalized direction of the placement X Axis. This is [1.0,0.0] if RefDirection is omitted. P[2]: The normalized direction of the placement Y Axis. This is a derived attribute and is orthogonal to P[1]. If RefDirection is omitted, it defaults to [0.0,1.0]	X

EXPRESS Specification

ENTITY IfcAxis2Placement2D
 SUBTYPE OF (IfcPlacement);
  RefDirection : OPTIONAL IfcDirection;
 DERIVE
  P : LIST [2:2] OF IfcDirection := IfcBuild2Axes(RefDirection);
 WHERE
  RefDirIs2D : (NOT (EXISTS (RefDirection))) OR (RefDirection.Dim = 2);
  LocationIs2D : SELFIfcPlacement.Location.Dim = 2;
END_ENTITY;