IfcGrid

Semantic definitions at the entity

Entity definition

IfcGrid ia a planar design grid defined in 3D space used as an aid in locating structural and design elements. The position of the grid (ObjectPlacement) is defined by a 3D coordinate system (and thereby the design grid can be used in plan, section or in any position relative to the world coordinate system). The position can be relative to the object placement of other products or grids. The XY plane of the 3D coordinate system is used to place the grid axes, which are 2D curves (e.g., line, circle, trimmed curve, polyline, or composite curve).

The inherited attributes Name and Description can be used to define a descriptive name of the grid and to indicate the grid's purpose. A grid is defined by (normally) two, or (in case of a triangular grid) three lists of grid axes. The following table shows some examples.


rectangular grid	radial grid	triangular grid

The grid axes, defined within the design grid, are those elements to which project objects will be placed relatively using the IfcGridPlacement.

HISTORY New entity in IFC Release 1.0.

Informal Proposition

Grid axes, which are referenced in different lists of axes (UAxes, VAxes, WAxes) shall not be parallel.
Grid axes should be defined such as there are no two grid axes which intersect twice.

Informal Proposition #2

left side: ambiguous intersections A1 and A2, a grid containing such grid axes is not a valid design grid.
right side: the conflict can be resolved by splitting one grid axis in a way, such as no ambiguous intersections exist.

Geometry Use Definitions

The geometric representation of IfcGrid is given by the IfcProductDefinitionShape, allowing geometric representations. Included are:

Local Placement

The local placement for IfcGrid is defined in its supertype IfcProduct. It is defined by the IfcLocalPlacement, which defines the local coordinate system that is referenced by all geometric representations.

The PlacementRelTo relationship of IfcLocalPlacement shall point (if given) to the local placement of the same IfcSpatialStructureElement, which is used in the ContainedInStructure inverse attribute, or to a spatial structure element at a higher level, referenced by that.
If the relative placement is not used, the absolute placement is defined within the world coordinate system.

Geometric Representations

Currently, the use of a 2D 'FootPrint' representation of type 'GeometricCurveSet' is supported.

GeometricCurveSet representation

The 2D geometric representation of IfcGrid is defined using the 'GeometricCurveSet' geometry. The following attribute values should be inserted

IfcShapeRepresentation.RepresentationIdentifier = 'FootPrint'.
IfcShapeRepresentation.RepresentationType = 'GeometricCurveSet' .

The following constraints apply to the 2D representation:

The IfcGeometricCurveSet shall be an (and the only) Item of the IfcShapeRepresentation. It should contain an IfcGeometricCurveSet containing subtypes of IfcCurve, each representing a grid axis. Applicable subtypes of IfcCurve are: IfcPolyline, IfcCircle, IfcTrimmedCurve (based on BaseCurve referencing IfcLine or IfcCircle).
Each subtype of IfcCurve may have a curve style assigned, using IfcAnnotationCurveOccurrence referencing IfcCurveStyle.
Optionally the grid axis labels may be added as IfcTextLiteral, and they may have text styles assigned, using IfcAnnotationTextOccurrence referencing IfcTextStyle.

Illustrations

	The IfcGrid defines a placement coordinate system using the ObjectPlacement. The XY plane of the coordinate system is used to place the 2D grid axes. The Representation of IfcGrid is defined using IfcProductRepresentation, referencing an IfcShapeRepresentation, that includes IfcGeometricCurveSet as Items. All grid axes are added as IfcPolyline to the IfcGeometricCurveSet.
	The attributes UAxes and VAxes define lists of IfcGridAxis within the context of the grid. Each instance of IfcGridAxis refers to the same instance of IfcCurve (here the subtype IfcPolyline) that is contained within the IfcGeometricCurveSet that represents the IfcGrid.

Attribute definitions

#	Attribute	Type	Cardinality	Description	A
8	UAxes	IfcGridAxis	~L[1:?]	List of grid axes defining the first row of grid lines.	X
9	VAxes	IfcGridAxis	~L[1:?]	List of grid axes defining the second row of grid lines.	X
10	WAxes	-		This attribute is out of scope for this model view definition and shall not be set.
	ContainedInStructure	IfcRelContainedInSpatialStructure @RelatedElements	S[0:1]	Relationship to a spatial structure element, to which the grid is primarily associated.

Formal Propositions

Rule	Description
WR41	The placement for the grid has to be given.

Inherited definitions from supertypes

Entity inheritance

Attribute inheritance

#	Attribute	Type	Cardinality	Description	A
IfcRoot
1	GlobalId	IfcGloballyUniqueId	[1:1]	Assignment of a globally unique identifier within the entire software world.	X
2	OwnerHistory	IfcOwnerHistory	[1:1]	Assignment of the information about the current ownership of that object, including owning actor, application, local identification and information captured about the recent changes of the object, NOTE: only the last modification in stored.	X
3	Name	IfcLabel	[0:1]	Optional name for use by the participating software systems or users. For some subtypes of IfcRoot the insertion of the Name attribute may be required. This would be enforced by a where rule.	X
4	Description	IfcText	[0:1]	Optional description, provided for exchanging informative comments.	X
IfcObjectDefinition
	IsDecomposedBy	IfcRelDecomposes @RelatingObject	S[0:?]	Reference to the decomposition relationship, that allows this object to be the composition of other objects. An object can be decomposed by several other objects.	X
	Decomposes	IfcRelDecomposes @RelatedObjects	S[0:1]	References to the decomposition relationship, that allows this object to be a part of the decomposition. An object can only be part of a single decomposition (to allow hierarchical strutures only).	X
	HasAssociations	IfcRelAssociates @RelatedObjects	S[0:?]	Reference to the relationship objects, that associates external references or other resource definitions to the object.. Examples are the association to library, documentation or classification.	X
IfcObject
5	ObjectType	IfcLabel	[0:1]	The type denotes a particular type that indicates the object further. The use has to be established at the level of instantiable subtypes. In particular it holds the user defined type, if the enumeration of the attribute PredefinedType is set to USERDEFINED.	X
	IsDefinedBy	IfcRelDefines @RelatedObjects	S[0:?]	Set of relationships to type or property (statically or dynamically defined) information that further define the object. In case of type information, the associated IfcTypeObject contains the specific information (or type, or style), that is common to all instances of IfcObject referring to the same type.	X
IfcProduct
6	ObjectPlacement	IfcObjectPlacement	[0:1]	Placement of the product in space, the placement can either be absolute (relative to the world coordinate system), relative (relative to the object placement of another product), or constraint (e.g. relative to grid axes). It is determined by the various subtypes of IfcObjectPlacement, which includes the axis placement information to determine the transformation for the object coordinate system.	X
7	Representation	IfcProductRepresentation	[0:1]	Reference to the representations of the product, being either a representation (IfcProductRepresentation) or as a special case a shape representations (IfcProductDefinitionShape). The product definition shape provides for multiple geometric representations of the shape property of the object within the same object coordinate system, defined by the object placement.	X
IfcGrid
8	UAxes	IfcGridAxis	~L[1:?]	List of grid axes defining the first row of grid lines.	X
9	VAxes	IfcGridAxis	~L[1:?]	List of grid axes defining the second row of grid lines.	X
10	WAxes	-		This attribute is out of scope for this model view definition and shall not be set.
	ContainedInStructure	IfcRelContainedInSpatialStructure @RelatedElements	S[0:1]	Relationship to a spatial structure element, to which the grid is primarily associated.

Definitions applying to AISC Model View

Instance diagram

Concept usage

Placement

The Placement concept applies to this entity.

Concept inheritance

#	Concept	Model View
IfcGrid
	Placement	AISC Model View

Formal representations

XSD Specification

 <xs:element name="IfcGrid" type="ifc:IfcGrid" substitutionGroup="ifc:IfcProduct" nillable="true"/>

 <xs:complexType name="IfcGrid">

  <xs:complexContent>

   <xs:extension base="ifc:IfcProduct">

    <xs:sequence>

     <xs:element name="UAxes">

      <xs:complexType>

       <xs:sequence>

        <xs:element ref="ifc:IfcGridAxis" maxOccurs="unbounded"/>

       </xs:sequence>

       <xs:attribute ref="ifc:itemType" fixed="ifc:IfcGridAxis"/>

       <xs:attribute ref="ifc:cType" fixed="list-unique"/>

       <xs:attribute ref="ifc:arraySize" use="optional"/>

      </xs:complexType>

     </xs:element>

     <xs:element name="VAxes">

      <xs:complexType>

       <xs:sequence>

        <xs:element ref="ifc:IfcGridAxis" maxOccurs="unbounded"/>

       </xs:sequence>

       <xs:attribute ref="ifc:itemType" fixed="ifc:IfcGridAxis"/>

       <xs:attribute ref="ifc:cType" fixed="list-unique"/>

       <xs:attribute ref="ifc:arraySize" use="optional"/>

      </xs:complexType>

     </xs:element>

    </xs:sequence>

   </xs:extension>

  </xs:complexContent>

 </xs:complexType>

EXPRESS Specification


ENTITY IfcGrid

 SUBTYPE OF (IfcProduct);

  UAxes : LIST [1:?] OF UNIQUE IfcGridAxis;

  VAxes : LIST [1:?] OF UNIQUE IfcGridAxis;

  WAxes : OPTIONAL LIST [1:?] OF UNIQUE IfcStrippedOptional;

 INVERSE

  ContainedInStructure : SET [0:1] OF IfcRelContainedInSpatialStructure FOR RelatedElements;

 WHERE

  WR41 : EXISTS(SELF\IfcProduct.ObjectPlacement);
END_ENTITY;

 EXPRESS-G diagram

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