5.4.3.1 IfcAlignment

5.4.3.1.1 Semantic definition

For the purposes of IFC the English term "alignment" defines three separate but closely interconnected concepts.

1. definition of a reference system for linear positioning
2. safeguarding and optimization of the movement of vehicles - kinematic perspective
3. geometric construction of roads, railway tracks or other linear infrastructure

Reference system for linear positioning

An alignment is used to define a reference system to position elements mainly for linear construction works, such as roads, rails, bridges, and others. The relative positioning along the alignment is defined by the linear referencing methodology.

Kinematic perspective

In the kinematic perspective, the focus is on the safe and optimized movement of a vehicle under the constraints induced by changes in the direction of the horizontal and the vertical layout.

Geometric perspective

In the geometric perspective the focus is on the proper placement of horizontal and vertical segments to connect certain points along a proposed path. A huge body of knowledge has been developed over a long period of time, in many aspects predating the availability of modern computers and their software.

Supported shape representations of IfcAlignment are:

• IfcCompositeCurve as a 2D horizontal alignment (represented by its horizontal alignment segments), without a vertical layout.
• IfcGradientCurve as a 3D horizontal and vertical alignment (represented by their alignment segments), without a cant layout.
• IfcSegmentedReferenceCurve as a 3D curve defined relative to an IfcGradientCurve to incorporate the application of cant.
• IfcOffsetCurveByDistances as a 2D or 3D curve defined relative to an IfcGradientCurve or another IfcOffsetCurveByDistances.
• IfcPolyline or IfcIndexedPolyCurve as a 3D alignment by a 3D polyline representation (such as coming from a survey).
• IfcPolyline or IfcIndexedPolyCurve as a 2D horizontal alignment by a 2D polyline representation (such as in very early planning phases or as a map representation).

State of the art in contemporary engineering Generally, contemporary engineering follows these steps when defining an alignment.

1. First, a horizontal layout in a properly projected plane is established.

2. In a second step, the vertical layout (i.e. sequence of segments with constant gradients, together with smoothing segments showing a variation in gradient) is added.

3. In the rail domain, in most cases a cant layout is added to the horizontal layout to compensate a part of the unwanted lateral acceleration.

4. In a final step, the proposed layout is checked against a defined set of rules, formulas and thresholds to guarantee the conformance against the regulation.

The sequence of steps might change from case to case and might be repeated one or more times to achieve the economic objectives and fulfill regulatory safety requirements.

Contemporary alignment design almost always implements a 2.5 dimension approach. The resulting and documented geometry might be very precise or just good enough to meet safety thresholds. This depends on factors like priorities of the management, date of the design - existing alignments might have been designed more then 50 years ago - or software tools used. Working with legacy data in a high precision BIM model requires a good understanding of these factors.

Distinction between business logic and geometry definition

The alignment concept is organised into two parts. These two parts work together, but they can also be used and exchanged independently.

1. Business logic of alignment
2. Geometry definition of alignment

Business logic: the IFC schema allows to describe an alignment using terminology and concepts that are as close as possible to business ones. It allows to describe the layouts that make up the alignment (i.e. horizontal, vertical, cant), their segments' structure and attributes. Also, the business logic part provides the anchor point for domain specific properties, such as design speed or cant deficiency.

Geometry definition: the IFC schema provides well established IFC geometric entities to represent the business concepts.

A mapping between the business logic and its geometry definition in IFC is described by the concept templates related to the alignment geometry.

5.4.3.1.2 Entity inheritance

1. • IfcRoot
2. • IfcObjectDefinition
   • IfcPropertyDefinition
   • IfcRelationship
3. • IfcObject
   • IfcContext
   • IfcTypeObject
4. • IfcProduct
   • IfcActor
   • IfcControl
   • IfcGroup
   • IfcProcess
   • IfcResource
5. • IfcPositioningElement
   • IfcAnnotation
   • IfcElement
   • IfcLinearElement
   • IfcPort
   • IfcSpatialElement
   • IfcStructuralActivity
   • IfcStructuralItem
6. • IfcLinearPositioningElement
   • IfcGrid
   • IfcReferent
7. • IfcAlignment

5.4.3.1.3 Attributes

#	Attribute	Type	Description
IfcRoot (4)
1	GlobalId	IfcGloballyUniqueId	Assignment of a globally unique identifier within the entire software world.
2	OwnerHistory	OPTIONAL IfcOwnerHistory	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,
3	Name	OPTIONAL IfcLabel	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.
4	Description	OPTIONAL IfcText	Optional description, provided for exchanging informative comments.
IfcObjectDefinition (7)
	HasAssignments	SET [0:?] OF IfcRelAssigns FOR RelatedObjects	Reference to the relationship objects, that assign (by an association relationship) other subtypes of IfcObject to this object instance. Examples are the association to products, processes, controls, resources or groups.
	Nests	SET [0:1] OF IfcRelNests FOR RelatedObjects	References to the decomposition relationship being a nesting. It determines that this object definition is a part within an ordered whole/part decomposition relationship. An object occurrence or type can only be part of a single decomposition (to allow hierarchical structures only).
	IsNestedBy	SET [0:?] OF IfcRelNests FOR RelatingObject	References to the decomposition relationship being a nesting. It determines that this object definition is the whole within an ordered whole/part decomposition relationship. An object or object type can be nested by several other objects (occurrences or types).
	HasContext	SET [0:1] OF IfcRelDeclares FOR RelatedDefinitions	References to the context providing context information such as project unit or representation context. It should only be asserted for the uppermost non-spatial object.
	IsDecomposedBy	SET [0:?] OF IfcRelAggregates FOR RelatingObject	References to the decomposition relationship being an aggregation. It determines that this object definition is whole within an unordered whole/part decomposition relationship. An object definition can be aggregated by several other objects (occurrences or parts).
	Decomposes	SET [0:1] OF IfcRelAggregates FOR RelatedObjects	References to the decomposition relationship being an aggregation. It determines that this object definition is a part within an unordered whole/part decomposition relationship. An object definition can only be part of a single decomposition (to allow hierarchical structures only).
	HasAssociations	SET [0:?] OF IfcRelAssociates FOR RelatedObjects	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.
IfcObject (5)
5	ObjectType	OPTIONAL IfcLabel	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 or when the concrete entity instantiated does not have a PredefinedType attribute. The latter is the case in some exceptional leaf classes and when instantiating IfcBuiltElement directly.
	IsDeclaredBy	SET [0:1] OF IfcRelDefinesByObject FOR RelatedObjects	Link to the relationship object pointing to the declaring object that provides the object definitions for this object occurrence. The declaring object has to be part of an object type decomposition. The associated IfcObject, or its subtypes, contains the specific information (as part of a type, or style, definition), that is common to all reflected instances of the declaring IfcObject, or its subtypes.
	Declares	SET [0:?] OF IfcRelDefinesByObject FOR RelatingObject	Link to the relationship object pointing to the reflected object(s) that receives the object definitions. The reflected object has to be part of an object occurrence decomposition. The associated IfcObject, or its subtypes, provides the specific information (as part of a type, or style, definition), that is common to all reflected instances of the declaring IfcObject, or its subtypes.
	IsTypedBy	SET [0:1] OF IfcRelDefinesByType FOR RelatedObjects	Set of relationships to the object type that provides the type definitions for this object occurrence. The then associated IfcTypeObject, or its subtypes, contains the specific information (or type, or style), that is common to all instances of IfcObject, or its subtypes, referring to the same type.
	IsDefinedBy	SET [0:?] OF IfcRelDefinesByProperties FOR RelatedObjects	Set of relationships to property set definitions attached to this object. Those statically or dynamically defined properties contain alphanumeric information content that further defines the object.
IfcProduct (5)
6	ObjectPlacement	OPTIONAL IfcObjectPlacement	This establishes the object coordinate system and 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 constrained (e.g. relative to grid axes, or to a linear positioning element). The type of placement is determined by the various subtypes of IfcObjectPlacement. An object placement must be provided if a representation is present.
7	Representation	OPTIONAL IfcProductRepresentation	Reference to the representations of the product, being either a representation (IfcProductRepresentation) or as a special case of a shape representation (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.
	ReferencedBy	SET [0:?] OF IfcRelAssignsToProduct FOR RelatingProduct	Reference to the IfcRelAssignsToProduct relationship, by which other products, processes, controls, resources or actors (as subtypes of IfcObjectDefinition) can be related to this product.
	PositionedRelativeTo	SET [0:?] OF IfcRelPositions FOR RelatedProducts	Reference to the IfcRelPositions relationship, which defines its relationship with a positioning element.
	ReferencedInStructures	SET [0:?] OF IfcRelReferencedInSpatialStructure FOR RelatedElements	Reference to the objectified relationship IfcRelReferencedInSpatialStructure may be used to relate a product to one or more spatial structure elements in addition to the one in which it is primarily contained.
IfcPositioningElement (2)
	ContainedInStructure	SET [0:1] OF IfcRelContainedInSpatialStructure FOR RelatedElements	Relationship to a spatial structure element, to which the positioning element is primarily associated.
	Positions	SET [0:?] OF IfcRelPositions FOR RelatingPositioningElement	No description available.
Click to show 23 hidden inherited attributes Click to hide 23 inherited attributes
IfcAlignment (1)
8	PredefinedType	OPTIONAL IfcAlignmentTypeEnum	A list of types to further identify the object. Some property sets may be specifically applicable to one of these types.

5.4.3.1.4 Property sets

• Pset_LinearReferencingMethod
  • LRMName
  • LRMType
  • UserDefinedLRMType
  • LRMUnit
  • LRMConstraint
• Pset_Risk
  • RiskName
  • RiskType
  • NatureOfRisk
  • RiskAssessmentMethodology
  • UnmitigatedRiskLikelihood
  • UnmitigatedRiskConsequence
  • UnmitigatedRiskSignificance
  • MitigationPlanned
  • MitigatedRiskLikelihood
  • MitigatedRiskConsequence
  • MitigatedRiskSignificance
  • MitigationProposed
  • AssociatedProduct
  • AssociatedActivity
  • AssociatedLocation
• Pset_Tolerance
  • ToleranceDescription
  • ToleranceBasis
  • OverallTolerance
  • HorizontalTolerance
  • OrthogonalTolerance
  • VerticalTolerance
  • PlanarFlatness
  • HorizontalFlatness
  • ElevationalFlatness
  • SideFlatness
  • OverallOrthogonality
  • HorizontalOrthogonality
  • OrthogonalOrthogonality
  • VerticalOrthogonality
  • OverallStraightness
  • HorizontalStraightness
  • OrthogonalStraightness
  • VerticalStraightness
• Pset_Uncertainty
  • UncertaintyBasis
  • UncertaintyDescription
  • HorizontalUncertainty
  • LinearUncertainty
  • OrthogonalUncertainty
  • VerticalUncertainty
• Qto_BodyGeometryValidation
  • GrossSurfaceArea
  • NetSurfaceArea
  • GrossVolume
  • NetVolume
  • SurfaceGenusBeforeFeatures
  • SurfaceGenusAfterFeatures

5.4.3.1.5 Concept usage

Concept	Usage	Description
IfcRoot (2)
Revision Control	General	Ownership, history, and merge state is captured using IfcOwnerHistory.
Software Identity	General	IfcRoot assigns the globally unique ID. In addition it may provide for a name and a description about the concept.
IfcObjectDefinition (9)
Classification Association	General	Any object occurrence or object type can have a reference to a specific classification reference, i.e. to a particular facet within a classification system.
Aggregation	General	No description available.
Approval Association	General	No description available.
Constraint Association	General	No description available.
Document Association	General	No description available.
Library Association	General	No description available.
Material Association	General	No description available.
Material Single	General	No description available.
Nesting	General	No description available.
IfcObject (5)
Object Predefined Type	General	No description available.
Object Typing	General	Any object occurrence can be typed by being assigned to a common object type utilizing this concept. A particular rule, restricting the applicable subtypes of IfcTypeObject that can be assigned, is introduced by overriding this concept at the level of subtypes of IfcObject. This concept can be applied to the following resources: IfcTypeObject
Object User Identity	General	An attribute Name and optionally Description can be used for all subypes of IfcObject. For those subtypes, that have an object type definition, such as IfcBeam - IfcBeamType, the common Name and optionally Description is associated with the object type.
Property Sets with Override	General	Any object occurrence can hold property sets, either directly at the object occurrence as element specific property sets, or at the object type, as type property sets. In this case, the properties that are provided to the object occurrence are the combinations of element specific and type properties. In case that the same property (within the same property set) is defined both in occurrence and type properties, the property value of the occurrence property overrides the property value of the type property.
Assignment to Group	General	No description available.
IfcProduct (18)
Body Geometry	General	The body or solid model geometric representation of an IfcProduct is typically defined using a Tessellation or Brep. Subtypes may provide recommendations on other representation types that may be used. The following attribute values for the IfcShapeRepresentation holding this geometric representation shall be used: IfcShapeRepresentation.RepresentationIdentifier = 'Body' IfcShapeRepresentation.RepresentationType = Typically 'Tessellation' or 'Brep'
Product Geometric Representation	General	The geometric representation of any IfcProduct is provided by the IfcProductDefinitionShape allowing multiple geometric representations. It uses the Product Placement concept utilizing IfcLocalPlacement to establish an object coordinate system, in which all geometric representations are founded.
Product Geometry Colour	General	No description available.
Product Geometry Layer	General	No description available.
Product Relative Positioning	General	If the IfcProduct Product Placement is placed relative to an IfcPositioningElement this relationship covers the information on which IfcPositioningElement positions the IfcProduct.
Product Span Positioning	General	No description available.
Box Geometry	General	No description available.
CoG Geometry	General	No description available.
Mapped Geometry	General	No description available.
Object Typing	General	This concept can be applied to the following resources: IfcTypeProduct
Product Local Placement	General	No description available.
Product Topology Representation	General	No description available.
Property Sets for Objects	General	This concept can be applied to the following resources: Pset_Risk Pset_Tolerance Pset_Uncertainty
Quantity Sets	General	This concept can be applied to the following resources: Qto_BodyGeometryValidation
Reference Geometry	General	No description available.
Reference SweptSolid Geometry	General	No description available.
Reference SweptSolid PolyCurve Geometry	General	No description available.
Reference Tessellation Geometry	General	No description available.
IfcPositioningElement (1)
FootPrint Geometry	General	No description available.
Click to show 35 hidden inherited concepts Click to hide 35 inherited concepts
IfcAlignment (12)
Alignment Aggregation To Project	General	Every IfcAlignment must be related to IfcProject using the IfcRelAggregates relationship - either directly or indirectly. The indirect case is when a child alignment is aggregated to a parent alignment. In this case, only the parent alignment must be related to IfcProject. Instances of IfcAlignment must not be related to spatial elements using IfcRelContainedInSpatialStructure.
Alignment Layouts	General	A single alignment may be described by one or more of the following layouts: a horizontal layout (IfcAlignmentHorizontal), defined in the x/y plane of the engineering coordinate system. an accompanying vertical layout (IfcAlignmentVertical), defined along the horizontal layout in the distance along / z coordinate space. an accompanying cant layout (IfcAlignmentCant), defined as lateral inclination along the horizontal layout. These 3 layouts may be used in different configurations. The most common are: Only Horizontal (H) layout Horizontal and Vertical (V) layouts Horizontal, Vertical and Cant (C) layouts Multiple Vertical layouts based on the same Horizontal Layout Multiple Vertical layouts based on the same Horizontal Layout, with a Cant layout applied (4) and (5) are used in scenarios where multiple alignments re-use the same horizontal layout definition. See Alignment Layout - Reusing Horizontal Layout and Alignment Geometry - Reusing Horizontal Layout for details on how to relate parent and child alignments in these cases.
Alignment Layout - Horizontal, Vertical and Cant	General	For configurations (1),(2),(3)
Alignment Layout - Reusing Horizontal Layout	General	For configurations (4),(5)
Alignment Geometry - Horizontal	General	For configuration (1)
Alignment Geometry - Horizontal and Vertical	General	For configuration (2)
Alignment Geometry - Horizontal, Vertical and Cant	General	For configuration (3)
Element Decomposition	General	In scenarios where multiple alignments re-use the same horizontal layout definition, these alignment can be aggregated in a parent-child relationship, to avoid redefining the horizontal layout. This is done using IfcRelAggregates. If the parent alignment has a geometry definition for its horizontal layout, then all the children can use this definition as BaseCurve to construct their own representations. This concept can be applied to the following resources: IfcAlignment
Alignment Spatial Reference	General	Alignments can be related to a spatial element using IfcRelReferencedInSpatialStructure.
Alignment Grouping	General	Alignments may be assigned to groups using IfcRelAssignsToGroup, where IfcGroup or subtypes may capture information common to multiple alignments.
Object Nesting	General	Alignments can nest IfcReferent's, such as stations or mileage points. These can be used as semantic entities holding information about locations along the alignment curve. This is done using the IfcRelNests relationship. This concept can be applied to the following resources: IfcReferent
Property Sets for Objects	General	This concept can be applied to the following resources: Pset_LinearReferencingMethod

5.4.3.1.6 Examples

• 

  Linear Placement Of Signal

• 

  Fixed Reference Swept Area Solid

• 

  Sectioned Solid Horizontal

5.4.3.1.7 Formal representation

ENTITY IfcAlignment
 SUBTYPE OF (IfcLinearPositioningElement);
	PredefinedType : OPTIONAL IfcAlignmentTypeEnum;
END_ENTITY;

5.4.3.1.8 References

• IfcLinearPositioningElement