Tuesday, February 28, 2012

Linking Revit Models: Origin to Origin

Revit has an origin point just like AutoCAD.  Revit calls this the "Project Base Point" and by default, it is hidden from us.  It's easy enough to find in a floor plan view by using the "Reveal Hidden Elements" tool. (the little light bulb in the view control bar)

The N/S, E/W, and Elevation coordinates of the project base point are relative to the "Survey Point".  By default these two points are coincident.  This project base point was moved 5 feet left and up for demonstration purposes.

When you link another Revit model into your "host" model, the "Import/Link RVT" dialog box has an option for "Positioning".  The default setting for this option is "Auto - Center to Center".  (I think it's the default simply due to alphabetical sorting.)  If you have ever used the rotate command in Revit with multiple items selected, you will understand how Revit calculates the "Center" of a selection set.  Using this option will apply that same center locating logic to an entire model.  Now, unless you are linking two identical models, it is very unlikely that the center points will align with the project base points.  Furthermore, your project "Center Point" will change as your model progresses and evolves.  Making it an ever moving target.  Not ideal for getting models to line up.

The more consistent and reliable option for positioning is to use the "Auto - Origin to Origin" option.

Using the "Origin to Origin" option will align the project base point of the linked model with the project base point in your "host" model.  Just like linking a CAD file at 0,0,0.  If you get in the habit of always using the "Origin to Origin" option you will avoid a vast majority of  issues with linked model alignment.

A common procedure would be a consultant linking the architectural model into their "host" model with the "Origin to Origin" positioning option before they begin modeling any geometry.  This allows all of their model elements to be properly aligned with all of the other disciplines.

Note:  This post ignores the scenario of "Shared Coordinates" which would necessitate the use of that positioning option if the project were set up using shared coordinates.

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