Linking CAD Files with Large Extents

While testing out some Civil 3D CAD links last week, I noticed another strange thing that was happening with some of the links. When the CAD link was reloaded (either manually or by closing and reopening the Revit file), it would actually move within the Revit file. The two images below show where the link started and where it would move.                     link_location01                         link_location02

It seems to be unrelated to the Civil 3D “splitting” effect, since it also occurs with a standard AutoCAD file. After some further testing, it seems to be caused by a combination of two things:

  1. a CAD file with “large extents” that does not have anything drawn at the CAD origin
  2. linking in that CAD file via any of the Manual options

In my testing, both of these had to occur for the link to move. If either of those conditions were not true, then the CAD file would not move. For example, I tested out two CAD files with the same large triangle. These two files have the same CAD origin, but one has a line drawn from the origin to the triangle. They were both linked in via Manual – Center. When reloaded, only the one without the line to the origin moved (the effect was the same for Manual – Origin and Manual – Base Point).

link_compare01          link_compare02

extents errorHowever, if you draw a line to (or at) the CAD origin, Revit will give you an error.

This will happen every time you reload the link, open your Revit file, or anyone else reloads your link. This can get obnoxious fast, so I would suggest not having anything drawn at the origin.

But strangely enough, the “large extents” that will cause a (Manual) link to move is related to a different error:

limits error

Steve Stafford posted about this a while ago on Revit OpEd, but essentially Revit has two extents/ limits thresholds. The largest is over 20 miles and will trigger the “Geometry in the file has extents greater than 20 miles…” The second is smaller and less specific, but it shows up as the “Imported Object Beyond Limits” warning. Steve guessed that it is around 8ish miles, but my testing put it closer to 10 miles. But almost any CAD survey is going to be larger than 8-10 miles.

Finally, I also tested what happened when multiple instances of the same link were linked in with different positioning. If the first CAD link was linked with a Manual positioning, then all of the later instances would also move, even if linked in with an Auto positioning. For example, I linked in three instances, first by Manual – Base point, then Auto – Center to Center, and then Auto – Origin to Origin.auto_origin and center

Interestingly, the first (Manual) instance affects how the next two come in. Typically, both Auto – Center to Center and Auto – Origin to Origin will link in centered on the Project Base Point (just like the image on the right).

Instead, Center to Center links near the first instance (but not Centered on anything) and Origin to Origin actually links in “correctly” relative to the origin. When reloaded, they all move/ shift south, so that the Origin to Origin link is where it usually links in, centered on the Project Base Point.

3 instances                        3 instances_move

It is some strange stuff, though it may be worth noting that a dwg link will move the same distance but in the opposite direction/ quadrant of where it is located in AutoCAD. And it does seem to only move once, for whatever that’s worth.

link_location_autocad      link_location02

So in conclusion, the best solution seems to be to always link CAD files Origin to Origin. Just be aware that this option also pins the link immediately by default. As a result of these findings I have also updated my post on Linking & Managing the Survey.

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