Revit, IFC and coordinate systems

The most frequently asked question about Revit IFC definitely has to do with coordinates - so why is it so complicated?

Both Revit and IFC have their own concept of coordinate systems and the handling of local and world coordinates, which will be briefly introduced in this article.

Coordinate systems in Revit

Revit has 4 reference points, which are all superimposed in a new project in the standard template - we are talking about the internal Revit origin (Revit Internal), the project base point (Project Base Point), survey point (Survey Point) and the shared coordinates (Shared Site), whose origin is not visible, but is displayed as a reference in the project base point or survey point:

The points can be made visible in any view via Visibility / Graphics under Property (Site).

There are different ways and possibilities to work with these points, so here are a few best practices from my point of view:

Internal Revit origin

This point is fixed and cannot be moved.

All model contents must be located within a radius of 16 km / 10 miles, otherwise the geometry of Revit can no longer be calculated and displayed correctly. This is not really a problem, as such large buildings are not processed in a Revit file anyway. Problems only occur if content is linked that lies outside this radius (often happens with CAD files from the surveyor, for example).

Project base point

Defines the project coordinate system and is usually placed on a building ceiling or axis intersection. In principle, the point can be moved, but if possible it should be left at its original location, as this simplifies other workflows (e.g. IFC export).

Survey point

Usually placed at a known point on the property, e.g. at the intersection of property lines, and often serves as the common coordination point for the surveyor.

The survey point can also be moved, either in a fixed or unfixed state, as it also establishes a reference between the shared coordinate system. The bracket on the survey point symbolizes whether the survey point is currently fixed to the shared coordinate system or detached from it:

• Moving the deleted survey point (after deleting the bracket symbol) moves the survey point to a new point on the property regardless of the shared coordinate system. As soon as a common coordinate system has been defined, the survey point should only be moved once it has been solved!
• Moving the fixed survey point also moves the coordinate system at the same time and should only be done after consultation with other project participants!

Shared coordinates

Are used to share the same coordinate system between different project files and/or systems - these can be local coordinate systems or GIS coordinate systems, allowing the project to be located in relation to a world coordinate system.

Here too, there are various ways to enter GIS coordinates in Revit: under Manage, the coordinates can either be retrieved from a file (Acquire Coordinates) or specified at a point (Specify Coordinates at Point).

The best way is to retrieve the coordinates from an already correctly located DWG, e.g. the surveyor's plan, as this also transfers the correct coordinate system:

After setting the shared coordinate system, the corresponding coordinates are reported back on the project base point and the survey point:

Dealing with height / rotation

As shown in the last screenshot, the rotation between project north and true north as well as the height (e.g. above n.N.v.) can also be specified. The height can be specified directly in the solved survey point, whereby both the project base point and the fixed survey point can then be moved again in space. It is important to know that the actual height offset between the internal origin and the selected reference point is relevant for the IFC export - this is clearly visible in a section or a side view, for example.

The rotation of the project north is well documented in the Revit Help: Help | Rotate Project North | Autodesk

IFC export settings

What happens during IFC export and what is the point of the long introduction? With IFC export, all selected reference points can be selected as the origin - the internal origin and the project base point can even be selected with project or geographical north, which can lead to confusion.

In addition, it is often wrongly assumed that the GIS coordinates are only given with the "Shared coordinates" option and are therefore the best setting, but this is not the case. The information about the GIS coordinate system is stored in the background in the IFC file, just as the origin only exists virtually in Revit and is used to calculate the shared coordinates of the project or survey point. This point also highlights the difference between BIM and GIS systems:

• A GIS system is designed for horizontal structures and therefore works with GIS coordinates to take account of the earth's curvature and the associated deviations
• A BIM system is designed for vertical structures (buildings) and works with a local project coordinate system, as the deviations due to the curvature of the earth are negligible. The project base point is simply located in the GIS system so that the building knows where it sits on the globe.

The IFC schema supports both approaches and, especially since IFC4, offers a very simple way to pass on both the local project base point of the BIM project and the GIS system - in Revit, the corresponding information is displayed in the export settings:

This means that the reference point selected during export does not determine whether GIS coordinates are exported or not - but merely defines the local coordinate system to which the building is aligned.

What is the difference between the individual reference points? After all, there are 6 options available in Revit:

Basically, for each point, its origin is taken and the model contents are exported in relation to this origin. To illustrate this, I will superimpose the export results of my test file, which looks like this:

The result looks like this:

What does that mean?

The project base point, i.e. the "official" zero point of the model, is always mapped to the zero point of the IFC Viewer - regardless of the method used for export. So if you export according to the survey point or the internal Revit origin, these are also offset by their corresponding distance from the project base point.

The Shared coordinates option is a special case - this is actually only recommended for a local shared location, but not when using the GIS coordinates. When exporting, Revit indicates that the selected origin is very far away from the building and recommends selecting a different reference point:

If this option is nevertheless used for export, it depends on the selected recipient software how it is displayed - some IFC viewers do not display the result at all (e.g. Open IFC Viewer), while others (BIM Collab Zoom) place it close to its own origin.

Conclusion / Recommendation

LOCAL BASE POINT: A local origin (in Revit ideally the project base point or survey point / internal origin) should always be selected for the export of BIM projects to IFC. This point should also be known to the other trades and, if possible, also be used for the IFC export.

COORDINATION BODIES: In any case, it is advisable to use coordination bodies, which are placed at the coordination point in all authoring solutions and exported as well - this allows you to visually check whether the models have been superimposed correctly or to correct any deviations.

REFERENCING IFC FILES IN REVIT. If IFC files are linked in Revit, the link is always "Origin to Origin", i.e. the origin selected during export is mapped to the internal Revit origin (this is also the reason for the initial recommendation to leave the project base point on the internal origin).

UNDERSTAND GEOREFERENCING: Georeferencing is embedded in IFC4 and should not serve as the origin of coordination in BIM projects, but provides additional information for embedding in GIS systems.

More recommendations on the topic of BIM coordination can also be found in the official modeling guide: modeling_guide_v1.1.pdf (autodesk.net)