IfcAlignment

Item	SPF	XML	Change	Description
IFC4x1 Final 4.1.0.0
IfcAlignment			ADDED
IFC4x2 Candidate 4.2.0.0
IfcAlignment
PositionedRelativeTo			ADDED
Positions			ADDED
IFC4x2 to IFC4x3_RC1
IfcAlignment
Axis			MODIFIED	Type changed from IfcCurve to IfcLinearAxisSelect.
ReferencedInStructures			ADDED
IFC4x3_RC1 to IFC4x3_RC2
IfcAlignment
Axis	X	X	MODIFIED	Type changed from IfcLinearAxisSelect to IfcCurve.
IFC4x3_RC2 to IFC4x3_RC3
IfcAlignment
PredefinedType	X	X	MODIFIED	Name changed from Axis to PredefinedType. Type changed from IfcCurve to IfcAlignmentTypeEnum. Instantiation changed to OPTIONAL.
PredefinedType	X		DELETED

Semantic definitions at the entity

Entity definition

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

defintion of a reference system for linear positioning
safeguarding and optimization of the movement of vehicles - kinematic perspective
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.

NOTE See ISO 19148 Geographic information – Linear referencing for general definitions about linear referencing.

Kinematic perspective

In the kinematic perspective 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.

State of the art in contemporary engineering

Contemporary engineering usually establishes first a horizontal layout in a properly projected plane.
In a second step the vertical profile (i.e. sequence of segments with constant gradients together with smoothing segments showing a variation in gradient) is added.
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.
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 modeling and IFC core geometry

According to IFC modeling principles alignment entities are organised in two large parts. The two parts work together, but they can also be used independently from each other

Business aspects of alignment
Representation with the IFC geometry resources

Business aspects of alignment: Here the focus is a on a schema structure as close to business terminology as possible. It is possible to have a very detailed segment structure with many domain specific properties attached. Examples for domain specific properties are design speed or cant deficiency.

Representation with IFC geometry resources: Here the focus is on using as much of the established IFC geometry entities as possible. A mapping between Business aspects and IFC geometry is proposed.

IFC modelling

In IFC a single alignment must have:

a horizontal alignment defined in the x/y plane of the engineering coordinate system

A single alignment may have:

an accompanying vertical alignment, defined along the horizontal alignment in the distance along / z coordinate space
a relative alignment, defined as a span within another alignment and/or at constant or variable offsets
a 3D alignment, either computed from the horizontal and vertical alignment, or extracted from geospatial data.

Alignments may be aggregated into referents (IfcReferent) or derivative alignments. Derivative alignments may be used to indicate dependent alignments, such as an alignment for a bridge that is relative to a parent alignment for a road, where the child IfcAlignment may have its shape representation set to IfcOffsetCurveByDistances that starts and ends at a span within the extent of the shape representation of the parent IfcAlignment.

Alignments may be assigned to groups using IfcRelAssignsToGroup, where IfcGroup or subtypes may capture information common to multiple alignments.

Supported shape representations of IfcAlignment are:

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

The RepresentationIdentifier shall always be set to 'Axis' and the RepresentationType shall be set to either 'Curve2D' or 'Curve3D' depending on if the referenced curve is 2- or 3-dimensional

NOTE Derivative specifications (Model View Definitions) may expand the above set to include additional supported curve types.

Attribute definitions

#	Attribute	Type	Cardinality	Description	G
8	PredefinedType	IfcAlignmentTypeEnum	?		X

Formal Propositions

Rule	Description
AlignmentRepresentation	Only IfcShapeRepresentation.RepresentationIdentifier='Axis' and IfcShapeRepresentation.RepresentationType='Curve2D' or 'Curve3D' are allowed. If both 'Curve2D' and 'Curve3D' are defined, then the horizontal alignment 2D curve shall be the basis of the 3D curve.

Inherited definitions from supertypes

Entity inheritance

Attribute inheritance

#	Attribute	Type	Cardinality	Description	G
IfcRoot
1	GlobalId	IfcGloballyUniqueId		Assignment of a globally unique identifier within the entire software world.	X
2	OwnerHistory	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, NOTE only the last modification in stored - either as addition, deletion or modification. IFC4 CHANGE The attribute has been changed to be OPTIONAL.	X
3	Name	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.	X
4	Description	IfcText	?	Optional description, provided for exchanging informative comments.	X
IfcObjectDefinition
	HasAssignments	IfcRelAssigns @RelatedObjects	S[0:?]	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.	X
	Nests	IfcRelNests @RelatedObjects	S[0:1]	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 strutures only). IFC4 CHANGE The inverse attribute datatype has been added and separated from Decomposes defined at IfcObjectDefinition.	X
	IsNestedBy	IfcRelNests @RelatingObject	S[0:?]	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). IFC4 CHANGE The inverse attribute datatype has been added and separated from IsDecomposedBy defined at IfcObjectDefinition.	X
	HasContext	IfcRelDeclares @RelatedDefinitions	S[0:1]	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. IFC4 CHANGE The inverse attribute datatype has been added.	X
	IsDecomposedBy	IfcRelAggregates @RelatingObject	S[0:?]	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 definitions can be aggregated by several other objects (occurrences or parts). IFC4 CHANGE The inverse attribute datatype has been changed from the supertype IfcRelDecomposes to subtype IfcRelAggregates.	X
	Decomposes	IfcRelAggregates @RelatedObjects	S[0:1]	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 definitions can only be part of a single decomposition (to allow hierarchical strutures only). IFC4 CHANGE The inverse attribute datatype has been changed from the supertype IfcRelDecomposes to subtype IfcRelAggregates.	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	?	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
	IsDeclaredBy	IfcRelDefinesByObject @RelatedObjects	S[0:1]	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. IFC4 CHANGE New inverse relationship, change made with upward compatibility for file based exchange.	X
	Declares	IfcRelDefinesByObject @RelatingObject	S[0:?]	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. IFC4 CHANGE New inverse relationship, change made with upward compatibility for file based exchange.	X
	IsTypedBy	IfcRelDefinesByType @RelatedObjects	S[0:1]	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. IFC4 CHANGE New inverse relationship, the link to IfcRelDefinesByType had previously be included in the inverse relationship IfcRelDefines. Change made with upward compatibility for file based exchange.	X
	IsDefinedBy	IfcRelDefinesByProperties @RelatedObjects	S[0:?]	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. IFC4 CHANGE The data type has been changed from IfcRelDefines to IfcRelDefinesByProperties with upward compatibility for file based exchange.	X
IfcProduct
6	ObjectPlacement	IfcObjectPlacement	?	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	?	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
	ReferencedBy	IfcRelAssignsToProduct @RelatingProduct	S[0:?]	Reference to the IfcRelAssignsToProduct relationship, by which other products, processes, controls, resources or actors (as subtypes of IfcObjectDefinition) can be related to this product.	X
	PositionedRelativeTo	IfcRelPositions @RelatedProducts	S[0:?]		X
	ReferencedInStructures	IfcRelReferencedInSpatialStructure @RelatedElements	S[0:?]		X
IfcPositioningElement
	ContainedInStructure	IfcRelContainedInSpatialStructure @RelatedElements	S[0:1]	Relationship to a spatial structure element, to which the positioning element is primarily associated. IFC2x CHANGE The inverse relationship has been added to IfcGrid with upward compatibility IFC4 CHANGE The inverse relationship has been promoted from IfcGrid to this new supertype with upward compatibility	X
	Positions	IfcRelPositions @RelatingPositioningElement	S[0:?]		X
IfcLinearPositioningElement
IfcAlignment
8	PredefinedType	IfcAlignmentTypeEnum	?		X

Definitions applying to General Usage

Instance diagram

IfcAlignment

Concept usage

Alignment Layout

The Alignment Layout concept template applies to this entity as shown in Table 30.

Table 30 — IfcAlignment Alignment Layout

Alignment Geometry Gradient

The Alignment Geometry Gradient concept template applies to this entity as shown in Table 31.

Table 31 — IfcAlignment Alignment Geometry Gradient

Referent Nesting

The Object Nesting concept template applies to this entity as shown in Table 32.

Type
IfcReferent

Table 32 — IfcAlignment Object Nesting

Alignment Geometry Cant

The Alignment Geometry Cant concept template applies to this entity as shown in Table 33.

Table 33 — IfcAlignment Alignment Geometry Cant

Alignment Aggregation To Project

The Alignment Aggregation To Project concept template applies to this entity as shown in Table 34.

Table 34 — IfcAlignment Alignment Aggregation To Project

Alignment Layout - Horizontal, Vertical and Cant

The Alignment Layout - Horizontal, Vertical and Cant concept template applies to this entity as shown in Table 35.

Table 35 — IfcAlignment Alignment Layout - Horizontal, Vertical and Cant

Alignment Layout - Reusing Horizontal Layout

The Alignment Layout - Reusing Horizontal Layout concept template applies to this entity as shown in Table 36.

Table 36 — IfcAlignment Alignment Layout - Reusing Horizontal Layout

Alignment Geometry - Horizontal

The Alignment Geometry - Horizontal concept template applies to this entity as shown in Table 37.

Table 37 — IfcAlignment Alignment Geometry - Horizontal

Alignment Geometry - Horizontal and Vertical

The Alignment Geometry - Horizontal and Vertical concept template applies to this entity as shown in Table 38.

Table 38 — IfcAlignment Alignment Geometry - Horizontal and Vertical

Alignment Geometry - Horizontal, Vertical and Cant

The Alignment Geometry - Horizontal, Vertical and Cant concept template applies to this entity as shown in Table 39.

Table 39 — IfcAlignment Alignment Geometry - Horizontal, Vertical and Cant

Alignment Spatial Reference

The Alignment Spatial Reference concept template applies to this entity as shown in Table 40.

RelatingStructure

Table 40 — IfcAlignment Alignment Spatial Reference

Alignment Grouping

The Alignment Grouping concept template applies to this entity as shown in Table 41.

IsAssigned

Table 41 — IfcAlignment Alignment Grouping

Property Sets for Objects

The Property Sets for Objects concept template applies to this entity as shown in Table 42.

PsetName	Properties
Pset_LinearReferencingMethod

Table 42 — IfcAlignment Property Sets for Objects

mvdXML Specification


<?xml version="1.0" encoding="utf-8"?>

<ConceptRoot xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" uuid="733a949d-1cc8-4d40-bd6a-3836e3c195dd" name="IfcAlignment" applicableRootEntity="IfcAlignment">

 <Applicability>

  <Template ref="425ea152-03b9-4561-a5c8-8bc3b50f9289" />

  <TemplateRules operator="and">

  </TemplateRules>

 </Applicability>

 <Concepts>

  <Concept uuid="e554c63e-acc9-473f-9469-6124fef84b4a" name="Alignment Layout">

   <Template ref="334e7f67-fd69-465f-b314-a2010a997465" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="1d7ba8a9-6245-46c9-8d2e-d02779fdb8ae" name="Alignment Geometry Gradient">

   <Template ref="c9be2304-1385-4c9f-9803-58cf95c53efd" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="1da6f9f0-6ffc-42f6-ba24-f16e4f6450a0" name="Referent Nesting">

   <Template ref="38ca8788-5e47-411c-8294-d56f9e9a5691" />

   <TemplateRules operator="and">

    <TemplateRule Parameters="Type[Type]=&apos;IfcReferent&apos;" />

   </TemplateRules>

  </Concept>

  <Concept uuid="5d6a08e0-2d4f-494f-afd6-782daef4b37f" name="Alignment Geometry Cant">

   <Template ref="3ae6a682-f042-45a1-8875-777b882183d4" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="cf2ad581-5f73-4f92-ae9d-24b391e1dd31" name="Alignment Aggregation To Project">

   <Template ref="56396e81-a5e7-5290-a13f-ed0aeaa6ec64" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="0f1d2e7a-b790-460d-917d-b39d861a9010" name="Alignment Layout - Horizontal, Vertical and Cant ">

   <Template ref="9f2220fe-98ec-5afd-82ff-3cd0b0bbcc02" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="d8c14fa6-4395-44fe-87c3-656e230ade60" name="Alignment Layout - Reusing Horizontal Layout">

   <Template ref="31c80d61-aa70-5fdc-a1e4-eb94e37885fa" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="868c7f90-c335-43d6-9f24-0686ad82a8f2" name="Alignment Geometry - Horizontal">

   <Template ref="d585f293-93af-5964-b9cd-2d5c30038185" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="bbbac2ee-0e90-4d6e-9bae-2608b9b0934d" name="Alignment Geometry - Horizontal and Vertical">

   <Template ref="550c3d22-f72d-59ef-a6cd-ff23b9c79c3f" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="6f89fa08-e83a-4d37-8897-039af4154f2c" name="Alignment Geometry - Horizontal, Vertical and Cant ">

   <Template ref="e338408d-8ac0-5f45-8f3f-4b30e332611c" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="6a306d5f-edd8-44d7-b6c0-89f628b982ad" name="Alignment Spatial Reference">

   <Template ref="11c6fc93-9a4f-5eb6-b012-67d1630a0547" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="dafe9e85-3900-46fc-8728-9a530cfb3450" name="Alignment Grouping">

   <Template ref="fda7c80d-f881-5af8-b892-3a0df84511c6" />

   <TemplateRules operator="and">

    <TemplateRule />

   </TemplateRules>

  </Concept>

  <Concept uuid="6e3dda07-be29-4546-aa16-e498dddc0291" name="Property Sets for Objects">

   <Template ref="f74255a6-0c0e-4f31-84ad-24981db62461" />

   <TemplateRules operator="and">

    <TemplateRule Parameters="PsetName[Value]=&apos;Pset_LinearReferencingMethod&apos;" />

   </TemplateRules>

  </Concept>

 </Concepts>

</ConceptRoot>

Concept inheritance

#	Concept	Template	Model View
IfcRoot
	Identity	Software Identity	General Usage
	Revision Control	Revision Control	General Usage
IfcObjectDefinition
	Classification Association	Classification Association	General Usage
	Assignment to Product	Assignment to Product	General Usage
IfcObject
	Object User Identity	Object User Identity	General Usage
	Object Predefined Type	Object Predefined Type	General Usage
	Object Typing	Object Typing	General Usage
	Property Sets with Override	Property Sets with Override	General Usage
IfcProduct
	Product Placement	Product Placement	General Usage
	Product Geometric Representation	Product Geometric Representation	General Usage
	Relative Position	Product Relative Positioning	General Usage
	Product Span Positioning	Product Span Positioning	General Usage
	Product Geometry Colour	Product Geometry Colour	General Usage
	Product Geometry Layer	Product Geometry Layer	General Usage
	Product Relative Placement	Product Relative Placement	General Usage
IfcPositioningElement
	Placement	Product Local Placement	General Usage
	Footprint Geometry	FootPrint Geometry	General Usage
IfcAlignment
	Alignment Layout	Alignment Layout	General Usage
	Alignment Geometry Gradient	Alignment Geometry Gradient	General Usage
	Referent Nesting	Object Nesting	General Usage
	Alignment Geometry Cant	Alignment Geometry Cant	General Usage
	Alignment Aggregation To Project	Alignment Aggregation To Project	General Usage
	Alignment Layout - Horizontal, Vertical and Cant	Alignment Layout - Horizontal, Vertical and Cant	General Usage
	Alignment Layout - Reusing Horizontal Layout	Alignment Layout - Reusing Horizontal Layout	General Usage
	Alignment Geometry - Horizontal	Alignment Geometry - Horizontal	General Usage
	Alignment Geometry - Horizontal and Vertical	Alignment Geometry - Horizontal and Vertical	General Usage
	Alignment Geometry - Horizontal, Vertical and Cant	Alignment Geometry - Horizontal, Vertical and Cant	General Usage
	Alignment Spatial Reference	Alignment Spatial Reference	General Usage
	Alignment Grouping	Alignment Grouping	General Usage
	Property Sets for Objects	Property Sets for Objects	General Usage

IfcAlignment

Semantic definitions at the entity

Inherited definitions from supertypes

Definitions applying to General Usage

IfcAlignment

Formal representations