I've seen a few people use this technique, but I have never really gotten my head around it until this week. I spent some time digging into Jack's papers on roundabouts from a few years ago, Jack's AU class and watching Sean's AU class on roundabouts and grading. They both use the idea of creating a temporary corridor which can then be used to drive design.
Anyone who has build a few corridors and knows how to use targets can usually map out a plan for modeling something somewhat complicated. It all comes down to how many alignments and profiles your are willing to manually create, design and babysit during edits. But what if you could make some dynamic pieces that gave you the information so you could either trace it, or target those pieces directly?
Let's look at a simple example. A bump out along a road needs to be graded at 2%. Now, I could probably pretty quickly make a profile that was 2% higher than my centerline profile, but what happens if I wanted to change it later? Plus, my target alignment has a widening, so it is stationed differently from my main alignment which can be annoying to manage and hard to visualize in your head.
One way to get started is to make a new assembly that is a copy of the one you will use on the final road corridor, only it has a LinkWidthAndSlope subassembly on one side that goes up at 2%.
When I build the temporary corridor, it looks like this:
Now, I make a surface from this corridor and sample it on to a profile view. Probably best to put it on its own profile view so you can see it fully stretched out, but you could also sample it on to the centerline profile view if you wanted.
Now, build an assembly for that region with the appropriate transition lane.
And build the final corridor using the offset alignment and the sampled surface profile as targets.
The nice thing about this is that if there is a change to your road centerline profile, you simply rebuild the temporary corridor and make sure the temporary corridor surface rebuilds, then rebuild your final corridor. Since it is targeting that dynamic surface profile, everything will update. Also, if you wanted to use 3% instead of 2%, you could just adjust the link in the subassembly.
The possibilities here are endless when you think of all of the manual profile chasing you do in your work- how much of it can we eliminate this way?
While this technique works in Civil 3D 2006, 2007, 2008 and 2009... Civil 3D 2010 makes it even more fun with dynamic offset alignments and widenings Here's my first attempt at an animated gif to show you how cool these are: