Sunday, September 21, 2008

DWG Import - Lines Nearly Vertical/Horizontal are Changed

Revit alters the lines in a imported cad file if they are very nearly horizontal or vertical. It has done this for quite some time. It isn't clear if it isn't a problem very often or if it isn't noticed very often. I'm reminded of this issue by a recent post at AUGI naturally.

In this instance a member wrote that a property line (in a cad import) whose bearing is 89 degrees 57 minutes and 56 seconds is altered by Revit to simply 90 degrees even. The cad file isn't "changed" but what we see of it in Revit is.

If you use Revit's Property Line tool and opt for the Table of Distances and Bearings method you'll find you can create a property boundary that is faithful to the documented bearing data.


And the dialog itself with the resulting property line.


Aah...Trust...a database or cad file is only as reliable as we believe we can trust it to be. Once upon a time people trusted that the earth was flat. That proved to be false, I think. I haven't been in space myself so I have to trust that the astronauts and scientists who say they have been are telling me the truth. Now I know that the moon is round but I don't know that it is in fact a globe because I've never seen the "back" of it. I've never seen it spinning. My life experience and acquired knowledge tells me it is a globe but assumption and belief doesn't prove it. Anyone who enjoys Robert Heinlein's books would recognize that last little bit.

Importing cad files and "seeing" nothing to cause us to doubt the accuracy of the import permits us to work with the belief that everything is fine, trustworthy. But when we discover an inaccuracy regardless how meaningful or meaningless the issue really is it places doubt in our minds. This doubt is usually worse than the actual problem we've found.

As we transition from 2D methods to 3D methods and embrace the promise of Building Information Modeling (BIM) we need to be able to trust the data. This is much bigger than just software. It has to do with each person that comes into contact with the data, the model. Our contact must be done with this issue of trust in mind so that we don't invite doubt. When doubt exists our work must stand up under close scrutiny.

4 comments:

DavidS said...

In my experience Revit's biggest flaw is its inability to handle non-orthogonal lines. This not only applies to lines just slightly off the orthogonal, but also lines at non-standard angles to orthogonal such as occurs when two wings of a building need to be parallel to two different roads which are not orthogonal to each other. This is compounded by the inability to set a section cut perpendicular to such a wing.

MRV said...

Well, that was my comment in AUGI, and I had never run across the "problem" before. In fact, I was importing entire subdivisions, and sketching the property lines over the import (without using a table). I never ran across a lot that had the "near orthogonal" lines, so I didn't see the problem.
Our trust in Revit definitely has changed since this "correction" has been brought to our attention!

Danny Polkinhorn said...

You must be a Fair Witness. "The house is white on this side." or something like that. Been 20 years since I read Stranger...

Steve said...

Rocket Ship Galileo is the one I was referring to but he reused themes between various stories a little bit. 20 years since you read Stranger...about 60 or so since he wrote it! He had a much more optimistic attitude about our travel into space. He probably would be a bit disappointed at how "far" we've gone?