How to avoid generating unnecessary triangles in between "platforms" for surface generation with breaklines

[soonhui]

I want to generate a few unconnected "platforms" ( As-Built Area) for my site design, as shown below.

I'm using following the steps here. I define the breaklines that constraints the platform ( platform shapes), and then I add those breaklines to a surface. Then a surface will appear with the platform shapes correctly appear ( defined by the breaklines) with the correct elevation.

The problem is that the unnecessary triangles must also appear in between the triangles due to the nature of Tin Surface, as shown below:

So now I have to manually delete those unwanted triangles which is very tedious.

Is there a better workflow? I just specify the breaklines, the elevations, but there is no need to manually delete the triangles from the surface? Can Grading do the job, if yes, then how?

I attach my files here for your perusal. The original file, and the final file. Note that for the final file, I have a lot of triangles that are not yet deleted so it doesn't fully resemble my final site.

[will_young]

I opened the file but I couldn't see any unnecessary triangles?

[soonhui]

@will_young , that's exactly my point. Like I said, I only want to generate the surface within the breaklines that I draw, as shown in the following image:

But as you can see, there are a lot of unnecessary triangles that connect between my featurelines/breaklines/platforms, right in the image that you screenshot. 

[will_young]

Ah ok, this is how Civil 3d creates the surface, there is no need to delete them. I know it looks wrong at first but it is correct. Some other 3d software can create a mesh with squares but Civil 3d uses triangles.

There are other surface display styles like points or contour lines if you are looking for a different way to represent in a drawing.

[soonhui]

@will_young , so exactly how do you propose to change the style ( or however) to do whatever I want, even just in terms of appearance?

[will_young]

Depending on your template you should have some view styles as below which can be accessed via Surface Properties. Have you installed the country kit?

Civil 3D Country Kits

[soonhui]

@will_young , maybe I misunderstand you, but exactly how does changing the style ( which is just appearance) change the underlying data? The extra triangles are just something I don't need, and I don't know how to get rid of them-- at data level-- unless I delete them manually one by one. 

[will_young]

Please read the below, it gives a good explanation about triangulation:

Autodesk Civil 3D Help | About Creating a TIN Surface | Autodesk

If you're really not keen on the triangles you can make a grid surface but I'm not sure you would get a good result from your input data, as below there are a lot of limiations.

Solved: What's the Point of Grid Surfaces? - Autodesk Community

[CodeDing · Accepted solution]

@soonhui ,

You need to perform these steps in this order once your full TIN has been created.

1) Add an OUTER Boundary to your first breakline area (either a feature line or polyline, as long as it's closed)

2) Add a SHOW Boundary to your second breakline area (use either closed feature line or polyline)

3) Add a SHOW Boundary to your third breakline area (use either closed feature line or polyline)

4) Add a SHOW Boundary to your fourth breakline area (use either closed feature line or polyline)

I believe this is what you are looking for:

(your top-right area did not have closed line, so triangles are acting bad)

Best,

~DD

[AuditInPaperSpace]

@soonhui you are right to not want triangles there. TIN surfaces have a boundary feature for a reason! 🙂 Seconding what @CodeDing said; That is the correct answer. Video tutorial here as well, if you prefer videos: https://youtu.be/ff80pNUCFkQ

It looks like some of your boundaries are arcs, so make sure the mid-ordinant distance you set for the boundary is as low as the mid-ordinate distance when you added the feature lines.

Note that there was a bug (they fixed it in C3D 2026) where Add Boundary always selects the bottom-most display order object (no matter what), so if you have a bunch of overlapping objects along your boundary (and not all of them are closed), keep that in mind. Sometimes you'll add a boundary and it will delete half the surface inside the boundary. 😄 If the boundary looks like it's doing what you want, then don't worry about it; You're good.

[ChrisRS]

Using grading objects would be a good idea here. They can be finicky.

Make sure all of your feature lines are on the same site.
Add a grading group to the site. Be sure to click Create automatic surface.

Add infill gradings to each of the areas.

(Do not add feature lines as break lines.)

A revised file is attached.

You can grade away from the pads with grading objects, but thar gives rounded slopes at the corners. If you are daylighting to an existing or graded surface you can do that with a grading object. The grading object portions are automatically added to the surface.

You can also do this with feature line based corridors.

[will_young]

Ah sorry I thought you meant you didn't want the triangles within the platform boundaries.

As the others have said, boundaries are what you need.

[soonhui]

@ChrisRS , what do you mean by " feature line based corridors"? Do you have an example for this?

[ChrisRS]

I am not a corridor expert, but I know this much:
Feature lines let you shape corridors more freely, without needing a full alignment and profile setup. They can be used to control elevation and transitions dynamically, making them great for detailed site design.

For an example and tutorial, check out this YouTube tutorial on using feature lines in Civil 3D corridors. It walks through the process step by step, making it easier to grasp. Let me know if you need more clarification!

I think you would find learning about graphing groups and grading objects beneficial. Less fussing with triangles and boundaries.

Good Luck!

I glanced at the video, but did not watch it all. I let Copilot write much of this.

[soonhui]

I got it figured out! Please ignore the below.

@ChrisRS , also on your grading group file ( the DWG that you attach), I tried to create another grading group, but I wasn't successful; the count is simply 0 as shown below:

Anything that I miss?