Linking Feature Lines to a Master Surface

Greetings from my new position at Engineered Efficiency!  I’m glad the folks here have let me join the party and I hope you’ll find my posts to be up to par with the high quality of past EE posts.

On to the subject at hand…

As you probably know, the ability to assign elevations from a surface to a feature line has been in Civil 3D for some time.  A big wish list item of mine has always been for that to be a dynamic relationship.  In other words, once the feature line gets its elevations from a surface, it would be nice if it would update those elevations to match any changes to the surface.  Well in Civil 3D 2009 ?!?!…..

…you still can’t do that (sorry, that was mean).  BUT, you can create a feature line that dynamically links to a profile so, in essence you can!

Why would you want to do this?  Well ages ago, Mr. Toby Jutras of Autodesk introduced me to the concept of creating a “master” surface that controls the elevations of the detailed geometry of your site.  The master surface covers a large area and is built from simple geometry.  A classic example is a simple panel-shaped surface that slopes across the entire site at, say 2% (like the gray lines in the image below).  This concept is extremely powerful and I’ve used it and taught it over and over and over.


Once you’ve got a master surface, you then draw out the detailed geometry of your site (parking lot perimeter curb, curb islands, etc.) and then simply assign elevations from your master surface to those objects.  What this allows you to do is set potentially hundreds of curb elevations in one fell swoop….a huge time saver.

Before discovering what I’m about to show you, I would have to re-assign all of those feature line elevations each time the master surface updated…kind of a drag.  But now, with the ability to link feature lines to profiles, I can make this relationship dynamic.

To test this theory out, basically what I did was draw a simple rectangle representing the outline of the parking lot and created an alignment out of it.  Then I drew a simple feature line and projected grading from it at 2% for 1000 feet (enough distance to completely cover up the parking lot).  The grading group automatically created its own surface, which I named Master.

Next I created an alignment out of the rectangle and sampled the Master surface.  Then, I used the command in the Grading menu to create a feature line from an alignment.  This command has the option of dynamically linking to a profile, which I selected.  What I’ve built is a relationship that looks something like this….

Master feature line or grading is updated=> Master surface is updated => Sampled profile of Master surface is updated => Feature line liked to profile of master surface is updated.

I can extend the chain by grading off of the linked feature line, which is what you’re seeing in the screen shots.  If you compare this screen shot…


…to the one above, you’ll see that I’ve rotated and moved the feature line, and changed the grading projection to 3%….that’s it.  Now the parking lot is sloping in the opposite direction and at a different grade.  No re-assigning, etc.  I did have to re-build the parking lot surface, but that only added on about 3 seconds, and voila!…a new parking lot model and updated contours.

Hope you like it!


  1. Ben Lund says:

    “Then, I used the command in the Grading menu to create a feature line from an alignment.”

    Is this a 2009 command?

  2. Yes it is. Grading > Create Feature Lines from Alignment.