Thursday, July 27, 2006

How to Corrupt a Civil 3D Drawing in One Easy Step

Want to corrupt a drawing beyond repair?

IGNORE
SITE TOPOLOGY

Ever heard of it? I doubt it.

The Civil 3D Essentials book barely mentions it.

The white paper on Large Subdivision design doesn't even have the WORD topology in it. And has little mention of the Civil 3D "site".

Yet, in order for parcels, alignments, feature lines and grading to work together well, you need to understand how they interact together, what interactions make sense, and how to separate them when necessary. They neither label correctly, nor behave correctly if you get this wrong. If they are really wrong, you will fatal error. If they are particularly bad, they will destroy your drawing. And I mean destroy- not even the jaws of life can get it open at that point.

I live, eat, breathe and sleep Civil 3D, and it wasn't until recently that I truly grasped what exactly was going on here and why it is so important.

My post about Parcel Rules Explained is really just a lesson in keeping parcel topology sound. I spent a little more time talking about Civil 3D "sites" in my post One Approach to Site Geometry .

More and more, I have been able to educate people on the ALIGNMENTS and PARCELS aspect of "site".

So now we are getting good. We are moving past parcels. We are doing feature lines and grading objects.

DID YOU KNOW that grading objects and feature lines ALSO interact with your site geometry?

Meaning: a feature line will bisect a parcel exactly the same way an alignment will if they are on the same site? That a grading object done within a parcel will do the same thing when they are on the same site?

How about this one. Extract a feature line from your corridor, and IT BECOMES THE SAME AS A PARCEL SEGMENT.

So now, our gorgeous site that we worked so hard to keep the alignments and parcels under control is HOSED because of feature lines.

Another thing- let's say you have two feature lines. One representing a ditch CL and one representing another ditch CL that crosses the first. Now, if they are on the same site, they both will not only get a PI at their crossing, but will also often fail to react to the grading tools because they "see" the other feature line.

Just like an alignment, if a feature line curls around itself a little bit, it seems to want to make a parcel. This appears to behave the same way as a "leaky bucket" (See parcel rules explained) and becomes fiercely unstable.

Suddenly yesterday while I was working with a client on a troublesome drawing, it finally hit me like a ton of bricks that over the past year and a half almost every single corrupt drawing that has come my way was very likely a victim of bad site geometry. Even the ones I didn't recognize as parcel or alignment problems.

But until you realize that, you are destined to hose drawings left right and center because it seems to be random.

I have a GIANT stack of work on my desk and a box full of email to sort through, and I would love to talk more about this, and I will. But I couldn't let another day go by without putting something up about this.

Until then, study the Parcel Rules and Site Geometry and keep your geometry clean and separated on different sites.

3 Comments:

Anonymous said...

Hi Dana;

Yes that sounds like a coup-de-etat to me also.

Now if you had spent some time explaining exactly what makes a Site "special", that might have helped those of us waiting in the "wings" for our turn at C3D.
;-)

Maybe that will be in Part 2 of this "thread"?

Don Reichle

kevin said...

And don't forget automatic PVI's has a bug in it until sp2 that will produce very interesting results...

http://discussion.autodesk.com/thread.jspa?messageID=5194312

(For some reason this post was giving OE fits, and people were only seeing half the post, hence the same thing was posted several times.)

Thomas Gail Haws said...

Way cool, Dana! We have a newly released "Lite" product called AutoCAD Civil that basically lacks the LDT bridge crutch, some hydrology tools, and maybe some data collector interface software.

Anyway, we are all going cold turkey form AutoCAD 2006 to Civil 2009, so this warning iwill be invaluable to us, and I will be following your blog further.

Tom Haws
Civil Engineer
Gilbert, AZ