Using Max Triangle Length to Make A Surface Boundary

A tip I picked up from an Autodesk University class was to use restrictions on surface triangulation length to help create a surface boundary.  This is useful for Existing Grade (EG) and Finished Grade (FG) surfaces; surfaces built with Point, Survey Figures, or Featurelines.  However, it does not work for surfaces built from a corridor model.  It’s a real shame too, because I think it would be most useful for corridor surfaces.  So here’s the trick…

In the below image you will see a FG surface that cuts back on itself.  When the computer triangulates between the Featurelines it covers a couple areas that we don’t want to have graded during construction.  This is not uncommon around the parameter of any EG or FG surface.

2010.01.04-Triangle issue

What we typically do to fix this is draw a polyline (pline) around the surface where you want it to cut-off and add that as an outer boundary to the surface.  This tick will help you create that outer boundary pline.

Notice how long the triangles are in these two areas.  With Civil 3D you can limit the maximum triangulation length temporally to help you generate the outer surface boundary.  Right-click on the surface and selectSurface Properties…‘.  Then go to the Definitions tab and expand the content under ‘Build‘ (see below).

2010.01.04-Triangle setting

After setting the maximum triangulation length, rebuild the surface and edit the Surface Style to show the surface ‘Border’ component.

2010.01.04-Srf Bndy ON

In the image below, notice how the triangulation / surface doesn’t span across the unwanted areas any longer.  Next, use the Surface–>Extract Objects from Surface command to create a pline from the surface Border that we turned on in the step above.

2010.01.04-Extract Bndy

When using this command it is key to un-check everything BUT ‘Border‘ and to select the border you want to extract.

2010.01.04-Extract Settings

Finally, edit the surface style again and turn off the Triangles and Border.  Then turn off the maximum triangulation limit and using the pline you just created, add it to the surface as an Outer Boundary.

[Note:  This will NOT work if your surface is built from a corridor model in any way.  Even a small piece coming from a corridor model will cause the surface to ignore the maximum triangulation setting.
There may also be more manipulation you need to do with the outer boundary pline created, but this gives you a good start.]

2010.01.04-Final Surface

THAT’S IT!, thanks and I hope this helps…


  1. Well, really a good tip, but I saw already at the site of “Sinc” 🙂

    • Thanks for sharing Sinc’s video, that’s a good follow up!

      The point of blogs isn’t to necessarily be the first to post something, Pedro, it’s to share the knowledge. There are a lot of people that don’t visit the all the sites like you do, and it’s obviously a good tip to share broadly.

      • Josh Petersen says:

        Well said James.

        I for one love blog posts like this. Easy to email around the office and a quick little read.

      • Sorry guys 🙁 and I just apologize about the way i said that, these are the problems of not using my native mother language.

        I was just wondering about. I know you all are frecuently users of autodesk forums and I thought you saw it already, and thats why i wouldnt think to email you with this tip…You’re one of my sources of knowledge 😉

        Happy New Year to all of people!! 🙂

  2. Yeah man, if you knew about this why didn’t you email me and let me know. 😉

    When I saw it during Peter Funk’s presentation I was all like “…woooow”. So I figured I would share it. 🙂

    • Yes, there were many oohs and aahs at Pfunk’s c3d trick party. AAMOF, i’ll be using that class as a source for one of my lunch and learn sessions we do. Good stuff all around!

  3. […] Length ~ 150′. This is a quick trick to deal with those surface edges. Actually, Mr. Spatz blogged this in detail over at earlier this month. Enough said i suppose, great […]

  4. I love this site. Everytime I come here I end up sitting here for WAY 2 LONG !! LOL Thanks admin. 🙂

  5. My says:


    Very interesting post. I would like to link back to it….

  6. John Kerr says:

    This is a nice option. You need to be careful with this option because the triangle length limit applies to the entire surface – not just the boundary. If you have a surface with large triangles in the interior, they’ll be removed just like the boundary vertices and you’ll be left with a holey surface.
    It would be nice if you could restrict the limitation to the edge of the surface (kind of like it used to work in Land Desktop and Softdesk and DCA).

  7. Mark says:

    Agreed, after you extract the Border you need to TURN OFF the max. triangle length option.