Wednesday, March 21, 2012

Interactive Terrain Shaping - Draping Feature Lines

One of the most powerful grading techniques that can be taught, in my opinion, is the technique of projecting, or draping a feature line to a surface.  This allows you to calculate many elevation points on a complex shape (such as a parking lot perimeter) in a matter of seconds.  Civil 3D can do this via the Elevations from Surface command, but one thing I always wished was that there could be a dynamic relationship between the feature line and the surface.  In other words, after using the Elevations from Surface command, I want the feature line to change automatically if the surface changes.  ITS grants this wish - kind of.

The command is called Drape Curve...

and it projects a feature line to a grading grid.

The feature line stays linked to that grid so any modifications to it, directly or indirectly, will cause the feature line to update - the wish that has been granted...kind of (read on).

Now you might call foul here saying "Hey that's not a surface!" and you'd be right.  But ITS has an answer for that too.  You can project a feature line to the original work canvas or you can create a new grid based on any surface by using the Create Working Element from Surface command.

The command is simple, launch it, pick a grading grid, then pick the surface.

Now you have a grading grid that can be used to drape a feature line.

Now for the "kind of" part.  As far as I can tell, a grading grid created from a surface is not dynamically linked to that surface.  I can't even find a way to update the grading grid after the surface is changed, other than creating a new grading grid. So there is no dynamic relationship between the feature line and the surface...bummer.  Maybe this is something that Autodesk can add in the future??  UPDATE - Be sure to read Smita's comment below about this very issue.


  1. There is a workaround to update the grading grid. Select the site boundary and move it by displacement (0,0,0). This will update all the outdated elements. (You may have to do this twice if it does not update everything the first time.) The dynamic update is not supported in this case because I was assuming that the surfaces will never change. However, it will be easy to support dynamic update of grids with edits to surfaces in future if that is what you are looking for. For now, you can use the workaround. Hope that helps.


  2. Thanks for replying Smita. Yes, I personally would want the behavior to be dynamic, in fact I fully expected it and was surprised when it didn't behave that way. Maybe others will weigh in as well.