Tuesday, February 07, 2006

Can I Trust You?

A common path for new Revit users is to import a dwg file and start tracing a project that got started in AutoCAD. Revit has a number of features that make this pretty easy to do. But just because it was drawn once in cad doesn't make it perfect or correct. I can't remember the last time I used a dwg file that didn't introduce some sort of error into the project I was involved in. What sort of errors am I talking about?

The old they sure look lined up but they aren't problem for one. This one happens alot. Two lines that are on either side of a corridor, forming an opening look lined up but if you look closely you'll find that they are off by a tenth of an inch or less. So when you use the PICK option to place walls you get two segments that are off slightly. The ALIGN command will fix it quick but you'd have to notice it first.

A sure fire hint that there is something wrong is when Revit doesn't offer a snap. If you pick an endpoint of a dwg line segment, move your cursor up to the other end and no SNAP icon is offered, BEWARE! The line segment is not aligned with the previous point.

Another is the line is almost nearly exactly plumb or horizontal, or almost exactly a clean angular value. Such as 89.001 degrees or .003 degrees. Sketching a Revit wall or line over such a dwg source will generate an error message like this one.
This simply means, "Gosh, this line is so close to a nice angle we thought we'd let you know that it doesn't make sense!"

So the next time you start tracing a dwg file to start your Revit model, be a little skeptical and cynical about the dwg source, even if you did the work yourself. Been there, done that!

Happy Tracing!!


Steve said...

Yikes! I loved the synopsis of this post at CAD Digest...

"Most Revit users will trace over an AutoCAD drawing because AutoCAD drawings almost always contain errors"

Haha! I had to laugh..yep we trace them BECAUSE they contain errors...ah the dangers of trying to summarize the essence of an article. They usually do pretty well though.

Anonymous said...

I have the opposite problem, I do draw a polyline with angles like 89.58° in the borders of the site, when I import the dwg into Revit, Revit forces to fit the angle to 90°(even it means to open the closed polylines), no warning just did it.
Therefore the acurate of dwg for measure areas is lost in the import action.

Steve said...

So you've imported a dwg file and you say that Revit has altered the angle of the dwg file? More likely the angular dimension style is using the Project Units setting and the tolerance is lower than the angle you are working with. This means that you'll get a rounded angular value.

If you sketch a line or wall over the angled dwg line using Snap Endpoint you should get a line at exactly the same angle. Revit may complain, but it will be exactly the same.

If you use the Pick option to place a line or wall same thing will occur.

Just because a dimension reads out the "wrong" value doesn't mean it isn't accurate, check the dimension style. Temporary dimensions will always show the true value regardless of what the dimension style does.

Hope this helps.

Anonymous said...

I'm experiencing a similar problem where a slightly skewed propertyline from AutoCAD was "corrected" to the nearest 1 degree when imported into Revit. I don't think it's a dimension style issue because where Revit had "corrected" the angle I see that the corners of the propertyline no longer meet.


Steve said...

So Revit has actually changed the dwg file? I imagine support would like to see this to examine if so. I'll have to try it out myself and see what happens.

Anonymous said...

I am having the same problem where Revit fixes my slight angle when imported into Revit from CAD