IfcAlignment is the IFC 4.3 entity for an alignment: the reference line of a road, railway, canal or other linear work. It holds the horizontal layout, the vertical profile and, for railways, the cant, so that elements such as bridges, signals and drainage can be positioned by their distance along it and exchanged in open BIM with that reference intact.
At a glance
| Schema | IFC 4.3 (ISO 16739-1:2024) |
| Describes | Horizontal alignment, vertical alignment, cant |
| Segments | Lines, circular arcs, transition curves; grades and vertical curves |
| Used for | Linear placement of elements along the alignment |
| Before IFC 4.3 | Alignments exchanged mostly in LandXML or vendor formats |
What it contains
| Part | Content |
|---|---|
| Horizontal | Straights, circular curves and transition curves in plan |
| Vertical | Constant grades and vertical curves along the horizontal |
| Cant | Rail superelevation along the alignment (railways) |
| Geometry and business logic | Both the design parameters (radius, length, grade) and the resulting curves |
| Placement | Elements placed by distance along and offset from the alignment |
How it is used on projects
Road and rail design tools export alignments to IFC 4.3, and coordination and checking tools read them, so structures and systems modelled by different teams can be placed against the same reference. Because elements can be located by linear placement, a model can answer "what is at chainage 2+340?" as well as "what is at these coordinates?". Georeferencing (IfcMapConversion and IfcProjectedCRS) ties the alignment to the real world.
Variations
- Road, rail and other domains use the same alignment concept with domain-specific parts (cant for rail).
- Several alignments may exist in one project: each carriageway, track or utility line.
- Software support for transition curve types varies; check that the curve types used survive export and import.
Common mistakes
- Exchanging an alignment as a 3D polyline only, losing its design parameters and stationing.
- Not georeferencing the IFC file, so the alignment can't be overlaid on GIS data.
- Assuming every tool supports every transition curve type in IFC 4.3.
- Placing elements by coordinates only, so they don't follow a revised alignment.
Where to read more
The IFC 4.3 documentation published by buildingSMART International (alignment and linear placement sections); ISO 16739-1:2024; and the buildingSMART infrastructure room's guidance and implementer agreements.