To Assign Linear Positioning

To assign a linear positioning reference

1. Click Add-ins tab IFC Infrastructure panel Assign Linear Positioning .
2. Select one or more entities in the draw to position.
3. Press Enter.
4. Select the alignment or profile to which the entities should align.

To remove a linear positioning reference

1. Click Add-ins tab IFC Infrastructure panel Remove Linear Positioning .
2. Select the entities to remove from the reference.
3. Select the alignment or profile to complete the reference removal.

About linear positioning in IFC

IFC incorporates linear positioning concepts through two distinct methods.

The first method involves a one-to-many positioning relationship between an IfcProduct (and its subclasses) and a linear positioning element, as detailed in the IFC4.3.2.0 Documentation 4.1.5.8 Product Relative Positioning. This positioning allows a user to locate the product in proximity to an IfcReferent, as described in the IFC4.3.2.0 Documentation 5.4.3.44 IfcReferent. This approach primarily serves to identify the location of a product, for example, a signal located at a specific station, chainage, or mileage along a designated alignment.

The second concept involves the relative placement of a product coordinate system and local geometry using a linear reference standard, as outlined in the IFC4.3.2.0 Documentation 4.1.7.4.2 Product Linear Placement. Instead of relying on Cartesian coordinates and a reference axis, an IfcPointByDistanceExpression can specify a coordinate system origin relative to a placement curve, as described in the IFC4.3.2.0 Documentation 8.9.3.48 IfcPointByDistanceExpression. The reference axis can be automatically determined by the tangency of the placement curve or explicitly specified.

It is important to note that in IFC, the equivalent Cartesian coordinates of the placement can also be specified as a fallback or checksum when defining a linear placement. The Civil 3D plugin includes these Cartesian coordinates for enhanced reliability.

The configuration of positioning and placement can now be specified by a user. A command is available to assign or remove the basis alignment or profile, which is stored on the Civil 3D (or AutoCAD) entity as an extended property set string containing the IFC export GlobalIds of the alignment or profile. These GlobalIds are separated by a semicolon (;). The GlobalIds can be consistently set using Dynamo, the API, or manually.

When the export is executed, the plugin examines each entity for this extended property set. If the alignment or profile is correctly identified within the drawing, the placement is computed for the first valid linear element, and a positioning relationship is established for all valid linear elements. An IfcReferent is generated at each linear location.