Forums >> Revit Building >> Technical Support >> Wiggly Lines when importing Topo into Revit

Alchemy68

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Hi All:  Does anyone know how to import a topographical survey (made in AutoCAD 3d, I think) into Revit in such a way as the lines do not appear "wiggly."  What I mean by "wiggly" is that the positions of the lines move around on the screen as you zoom in and out...very weird and errie and makes it difficult to have confidence in the revit linework you are overlaying on top since the underlaid lines seem to change position based upon the level of zoom.

The lines don't appear wiggly when I open the file in AutoCAD.  Just when I open it in Revit.  I tried "flattening" the autocad file prior to importing it into Revit and that didn't help.

Any sage advice is appreciated.

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WWHub

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You are far better off creating topography from the CAD.  Don't just use the CAD.

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Alchemy68

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I wish it were that simple.  While I can convert the topo into a Revit toposurface easily, the other aspects of the survey such as property lines, existing sidewalks and improvements, etc. are "wiggly," i.e., they mysteriously shift their locations on the screen depending upon zoom...a weird distortion...I know this might be a very difficult phenomenon to explain unless you've seen it before.  At any rate, I need to retain the CAD survey import as both an underlay and guide for the improvements and it's very difficult to snap to these existing lines when they are constantly shifting position.

So has anyone experienced this before and discovered the fix?

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WWHub

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This "issue" (I don't consider ir to be one) has been discussed many times here and elsewhere.  It is caused because of the process used and the differences between CAD and Revit.

Cad defines items using very, very small tolerances.  Revit limits that definition to 1/256".  Revit always likes to work in parallel angles.  So a line in CAD has two endpoints at say x.xxxxx , y.yyyyyy (I don't remember how many places) but I think Revit has a starting point, angle and length.  Parallel lines in Revit will have the same exact angle whereas CAD doesn't give a darn about angle.  This is why you can't always dimension two parallel lines imported from cad.   Zooming in exacerbates this differential.

So in your case, what do you do?

1. Leave the site to the Civil guys. << Best Option
2. Revit's property lines work perfectly so don't trace.
3. Use the import to create the CAD topography.
4. If you must show the CAD site, turn off layers like topography and use Revit's.
5. If you need to 'trace' elements like sidewalk's & parking then: If just for modeling, then trace them and don't worry about zoom in movements.  If they must be dimensioned, then locate and 'model' them correctly in Revit.  You can keep the CAD on as Reference but place them dimensionally correct from Revit objects.  << When using these options, the result will mean you turn CAD off.

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Alchemy68

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Thanks for your reply.

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Alchemy68

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The problem was with the AutoCAD extents of the original file (huge extents)...some of the objects on the survey were miles apart.  After I deleted the outlying elements from the original survey and decreased the AutoCAD limits, it imported into revit with no wiggly lines.  Problem solved.

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WWHub

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Yes, I kind of suspected the big extends on the CAD but I failed to mention that.  But in any case - look at what I suggest for process.  Either use the cad and don't change anything or use revit.  Tracing CAD always leads to problems.

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andy777bennett

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I have the same problem with no large extents. Maybe a half mile. All I want to do is show the stupid CAD in one floor plan. It is some landscape, not topo. The stupid thing dances like a 3-year old is drawing it. 

-CAD has no large extents (i unhid all layers to check)

-no 3d elements (I rotated and deleted all non flat things)

-scale is correct

-tried import and link, both same result

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WWHub

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I often wite out a new file from CAD selection what I want and defining a new origin.

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