Coordinating between Revit and Tekla using IFC

On several projects recently we've encountered issues when linking IFC's generated from Tekla Structures into Revit where the two files simply do not align depsite the IFC showing the correct geographical location and linking in fine into other software such as Solibri and Navisworks.

The problem lies in how Revit links in the IFC's. When linking an RVT or DWG file Revit provides options for the coordinate system you want to use to align the models, usually Center to Center, Origin to Origin, Shared Coordinates or Project Base Point to Project Base Point. For IFC's however we get none of that and instead Revit defaults to using Origin to Origin.

So the first thing is, what is and where is the 'Revit Origin'? This is the point the Revit uses to store all of it's geometric information, if you have used Dynamo and dealt with coordinates you will have seen the raw coordinates of this system. Typically though the user is never really aware of it as everything is simply modeled relative to other elements which arguably makes life easy for the user. To locate the origin point, you need to turn on the Project Base Point (PBP), un-clip it, right click and select 'Move to Startup Location' - this point is the Revit Origin. Typically on projects we will mark this with two grid lines labelled XX and YY for clarity. In many cases, depending on what information was available when the model was set up, the location of the Revit Origin will be completely arbitrary.

Okay so what about Tekla? Well it makes finding the location of it's origin much easier - it highlights it with a nice clear origin marker;

Again the exact location of this in the model will vary depending on how the Tekla model has been built but from my experience it tends to be located at the lower left hand grid intersection.

Now if these two origin points are different then the exported IFC from Tekla will not come in in the correct position in Revit. So how do we resolve this? Well fortunately with Tekla we can define different coordinate systems within the project and we can set one of these up to match the Revit Origin (and coordinate system) and then use that to generate our IFC.

The first step, after ensuring the PBP has been reset to its startup location, is to establish the offset between the two origin points (Tekla - Revit). This really needs to be done by the author of the Revit model and they will need to know the location of the Tekla origin point, if this is on a grid intersection then that's easy. In this example our Tekla Origin is at grid intersection P01.

The second bit of information we need are the national grid coordinates of the Revit Origin. To obtain this simply click on the PBP and record the values shown.

Once this information has been gathered we can see about setting up our coordinate system in Tekla. In the file menu select Project Properties and then Base Points. In the resulting dialogue box fill in the following fields.

If you generate the IFC at this point using this new coordinate system it will align perfectly in Revit. Job done you might think, but sadly, not. Most other packages that read IFC's such as Navisworks and Solibri make use of the IFC's site coordinates and North rotation. So if you were to import this into either of these packages its location would be 'correct' but the rotation would be off. The upshot is the IFC will either import correctly into Revit and wrong elsewhere or vice versa - you can't have both.

Because we find it easy to rotate things accurately in Revit we generally find it makes sense to have the IFC correct in terms of having the correct north rotation and fix this in Revit. Therefore the last thing you need to do is add in the 'Angle to North'. Ideally this will be a nice round number, or at least set to a specific number of decimals and easily communicated between team members / recorded in the BIM Execution Plan (BEP). Whatever value is set by the author of the Tekla file, this needs to be communicated back to the author of the Revit file. In this instance we have a rotation of 43.09 deg.

Now that all the data is complete* we can generate the IFC. Go to file, export and select IFC. Select the relevant settings (this *should* be in your BEP) making sure to select your new coordinate system for the 'Location by' field.

When this new IFC is linked into Revit it will be rotated about the Revit Origin. To fix this select the linked file in a view where the PBP is visible and then rotate it by the agreed amount, again in this instance 43.09deg. It should now align correctly with the Revit model.

*Typically we generate our IFC's using the MVD IFC2x3 Coordination View which doesn't include site elevation. If this is the standard for the project then there is no need to set the Z value in Tekla when setting up the coordinate system. However if all IFC's are being generated to include site elevation then this will need to be added as well. In this case when the file is imported into Revit it will also need to be dropped by a corresponding amount to correctly align the levels.

***Update***

Depending on the model setup we have noticed some issues with site levels when the IFC's are brought into Revit. If the linked IFC is 20m higher than it should be then when setting up the base point set the Z coordinate to +20m and (oddly perhaps) this will pull the model back down to the correct location.

Disclaimer : I'm not an expert in terminology I just like to figure out how to get the job done. If anyone notices anything incorrect in the terminology I've used in this post please let me know and I will correct it accordingly.