Questions about IFC Site in IFC 4.0

Hello there. Im currently working on a open BIM project with Revit where im stuck with what seems like a technicall limitation of IFC.
Here is the issue: I modell separate Topographies in Revit and need to export it as IfcSite but the topographies themselves have different Propertysets.
IFC though only recognizes 1 Element as IfcSite always merging every piece of topography together into one element.
Now my question first is related to the Ifc Standard website for IfcSite:
(Sadly I cant include Links or Pictures the forum says so youd have to check it up on the
standards.buildingsmart(dot)org website
It says in the first part that a IfcSite can be split into : Complex, Element and Partial sites though trying it, it didnt separate them at all.
Anyone knows about this inherited property how it works or if it even splits the topography in the IFC?
Or is there any other way to make a IfcSite split up into more parts in the same file?
Thanks in advance for every piece of info as i havent figured it out how it works at all.

Hello Kristof,
I work with Allplan and haven’t any problem to have several IfcSite in one project, as well as several buildings (IfcBuilding) on every site.
Here is a simple example - one IfcProject, four IfcSite and every IfcSite contains one or two IfcGeographicElement(s). For every IfcGeographicElement I can apply either same properties sets or make them different - its up to my own decision.
Here is a simple example (I didn’t input any special properties here, except Name), below, in IFC4.3.


TopoSites.ifc (9.9 KB)

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P.S.: here is in IFC4.0, I missed that you were asking for IFC4.0, result is the same:

TopoSites_IFC4.0.ifc (9.9 KB)

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Basic geometry comes in at my end.

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This misunderstanding creates a lot of frustration for Vectorworks users. I have removed IfcSite as data mapping and replaced it with IfcGeographicElement for IFC4 and IfcBuildingElementProxy for IFC2x3 or later. USERDEFINED type for the different Site Models can be set to SITEMODEL or TERRAIN_SURFACE. There is an old implementation agreement for IFC2x3 from 2007 that’s states that only one IfcSite should be used. IfcSite is not an object. IfcSite represents the outer boundary of the project or 3D Space. Site Model TIN surface is delivered as IfcGeographicElement or IfcBuildingElementProxy and not as IfcSite in my IFC files:)

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Interesting. Thank you for all the answears. I thought that the IfcSite itself could be split into those subcategories creating smaller separate parts. I didnt know about the IfcGeographicElement.
I thought Id need to export it as IfcElementCompositionEnum which is Inherited from IfcSpatialStructureElement (Which is the supertype of IfcSite) with instead of Enum having the 3 Options - COMPLEX; PARTIAL; ELEMENT; which I thought will split my IfcSite into those categories creating separate topographies.

@Kristof1995 I think there needs to be some clarification here:

  • You can have multiple IfcSites in a project. The fact Revit cannot do this is a Revit specific limitation.
  • If you export things out of Revit under IfcSite, these are invalidly created as if they were elements, not actual spatial elements. This is simply invalid practice and unfortunately common with Revit. Examples include people exporting a tree as an IfcSite, or an external bollard or street furniture as an IfcSite. It’s not correct.
  • IfcSite can contain geometry itself. It is not strictly necessary to then contain an IfcGeographicElement within it. There is nothing wrong with the site itself having geometry. Sites can have properties applied directly to them.
  • IfcSite can be split up into subsites, using the “COMPLEX > ELEMENT > PARTIAL” hierarchy you’ve noted. You can indeed split it to subcategories to create smaller parts.