# 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:

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.

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.

## 5.4.3.1.4 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

## 5.4.3.1.7 Formal representation

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