Just wondering if anyone ever had issues with linked in CAD files moving for no reason?
This issue deals once again with Shared Coordinate Systems and True North.
Many of our esteemed colleagues have written in depth explanations about how Shared Coordinate System works and how it relates to Revit's Project origin.
So here are the things that we know about specifying a shared coordinate of CAD files a large distance away, and have been talked about on the forums for quite some time.
1. Build your Revit Geometry Close to the Revit Origin. Revit doesn't like large numbers when it does it's calculations.
2. When inserting your CAD site plan file for the first time, drop in Center to Center. Move/ Rotate the CAD files in both plan and elevation, to be oriented to the Project North.
3. After Placement use the acquire coordinates tool from Revit Tools menu. Upon acquiring the coordinates, Revit will take the following action; revit will assume the XY (0,0,0) WCS location of the CAD fileas the shared coordinate system, wherein the Y axis is True North.
4. We only need to do this once, because everytime you insert a new cad file, if it doesn't share coordinates with Revit, then Revit will match the WCS of the CAD file to the Shared Coordinate.
This sounds all fine and dandy, and looks great on paper. I even did an entire discussion at a Chicago Revit User's Group on the topic.
Now here's the onion. Per Wes McCauley's post on the topic:
http://forums.augi.com/showthread.ph...ghlight=origin
What I described in steps 1-4 above runs in tandem with Mr. McCauleys post, but herein lies the problem. Revit never really gets away from hating large numbers. So if you have a site survey file wherein your site scope is a large distance away from your acquired Revit Shared Coordinate system, Revit will hiccup and lose the position of the CAD file upon File open and/or Manage Links reload. This appears to be completely random. Sometimes it shifts left, sometimes up, sometimes down, sometimes diagonal. It really doesn't seem to have a rhyme or reason to its relocation. When it does this, it also loses the object style settings of your import object dialogue, such as specific CAD Layers you set to be grey. What is really upsetting is that it will behave itself for weeks on end and then all of a sudden the night of your submission deadlines, all your CAD site information flys off in Random Directions. This takes place no matter which methodology you use to save the shared coordinate system into your CAD file, or even if you rely on the default WCS placement to the Shared Coordinate
Try correcting this everytime you issue, and you will surely want to do a triple gainer off the nearest balcony.
For a while I sucked it up and manually fixed the CAD files everytime Revit burped, however I began to notice a trend. It appeared that upon Revit reloading the CAD file it would only shift under the following condition:
-You have added or deleted a large chunk of CAD data, near the extents of your original drawing. When this takes place, upon Revit reloading the CAD file, Revit will decide to re-orient your CAD drawings to what suspiciously started to look like a Center to Center placement, i.e. if you added a large chunk of data to the north of your project, everything would shift down to be on the new center. If you added a large chunk of data to the Southwest of your drawing, it would relocate diagonally to the right and up. The random distance accredited to how much volume of information went beyond the original extents of your CAD file.
So the solution......I think it works.....none of my bloody CAD files have moved yet, and as I write this a major submissions is only hours 12 hours away.
-Draw a bounding box that is larger then the extents of your site cad file. Instruct your users to never draft cad elements outside of the provided box. This in turn will cause Revit to never see the change in the extents of the CAD file linked in, thus causing for a happy CAD file that stays where it is supposed to be. Looks good on paper, I think!
Now for the caviat... optimize the size of your bounding box. There is a sweet spot. If your bounding Box is rediculously large, then Revit will burp and forces the link to come in on center, even if you have the shared coordinate up and running. If your bounding box is too small then you are not providing enough space to add additional content and data.
Unfortunately, it's too early for me to say that this work around is a sure thing. I would love for fellow AUGI members to test my findings and comment.
--Regards
A Very Sleepy SOM Revit Guru