How To Guide: Combining (merging) Surfaces

October 30, 2017 By Leo Lavayen Leave a comment

The question comes up often on how to merge surfaces.  Autodesk has an 11 step process listed on the web, but I found it only applies to particular scenarios and doesn’t fully explain other options.         

The process is a simple paste to a COMBO surface when the need is to combine initial surface (EG in dashed RED) and supplemental surface (FG in solid BLUE), illustrated below.  When surfaces daylight, the tie-in is smooth, the workflow to create voided (donut hole) to then fill it is unnecessary.

image 1.jpg

However, the situation changes when the initial surface (EG from contours in dashed GRAY) and supplemental data (from survey in GREEN and/or RED) vary in elevations,

image 2.jpg

The issue is a rough transition, sometimes causing steep faces and ugly contours, shown in the point to point triangulation TIN created (shown in BLUE linework below).

image 3.jpg

To successfully achieve smooth contours from one surface to the next, you need to understand how Civil 3D behaves. 

Merging Surfaces (short version):

  • Surface Choice: keep a SINGLE Surface (edit directly) Creating NEW Surface to host edits.
  • Outline Refinement Area: create polyline around the area, set elevations to surface and add to surface as break line.
  • Clear out Refinement Area: to Delete TIN Points (causing planar area) Delete TIN Lines (creating a void/hole).
  • Adding New Data: paste data directly to fill area creating a new surface with data, then pasting and filling the area.


Merging Surfaces (long version):

#1 Surface Options

Option 1:

Keep a Single Surface to edit directly

Option 2:

Create New Surface to Host Edits

If you do not need to keep the initial conditions, you can make the changes directly to the initial surface and move on.


This may be the only option if the initial surface is already referenced for design.

If you would like to keep the initial surface as is, and work on the secondary surface you must:

1.      Create NEW Surface

2.      Paste the initial surface into it.

3.      Edit the Newly created surface


Wise option to keep the initial surface intact, if we later need to see or compare.


This option requires forethought in design and have the “Pasted” version be the one from which we start our design.


#2 Create Bounded Refinement Area at Surface

Regardless of the method chosen above, you are going to outline the Refinement Area to clear out the inside later:

1.      First, Using a 2D Polyline trace around the area and close it.  As you sketch this area, consider how large of a gap you want the software to “blend” the elevation differences.

 image 4-1.jpg2.      Next, from the ribbon Modify tab > Edit Elevation panel > use Elevations from Surface command, to set vertices to initial surface elevation:

image 5-1.jpg

3.      Don’t forget to select the Insert intermediate grade breaks points checkbox, to drape line on terrain

 image 6-1.jpg

4.      Once complete the polylines appear on surface with added vertices at terrain breaks

    image 7-1.jpg


#3 Add Created Bounding Area

Next, you assign the created bounded linework as a breakline to the surface. 

1.      Select the surface that hosts the edits, and in the contextual ribbon > from the Modify panel > expand the Add Data flyout > select Breaklines button

 image 8-1.jpg

2.      Make sure to set the Type as Standard, add Description as needed.

 image 9.jpg

Once complete your surfaces go FROM this:

 image 10.jpg

TO a supplemented scenario, with extra triangulation around the boundary. 

 image 11.jpg

This is used next as a “cookie cutter,” ensuring that terrain remains unchanged up to the boundary.


#4 Clearing out Refinement Area

(Do NOT use Hide Boundary)

Scenario 1: Delete TIN Points

button line.jpg

Results are blended triangularization within area

Scenario 2: Delete TIN Lines

button line 2.jpg Result is a void/hole within the area seen in profile

 image 12.jpg

 image 13.jpg

#5 Adding New Data

Scenario 1:

Where TIN Points were deleted within the boundary, causing blended areas.


Filled in with Supplemental Data directly.

In this case, you work from the previously edited surface “Definition” collection.  Add directly to surface supplemental data.  In the example below a point group was used that came from field verified data.

image 14.jpg

Scenario 2:

Where TIN Lines were deleted within the boundary, resulting in a void.


Filled in area with secondary Surface built from supplemental data.

1.      Create a New Surface

2.      Add Supplemental data to the NEW Surface

3.      Edit NEW surface as necessary

4.      Paste NEW surface to previously edited surface to fill in void

image 15-1.jpg

Results are not as expected, as there is a gap in between Pasted TIN and Boundary Edge.  To get over this is, the prosses is simple:

1.      Draw and Add an exterior boundary around the outside perimeter of the edited surface.

 Image 16.png

2.      Make sure to set Type to Outer

image 17.jpg

3.      This forces triangulation to create a “blend.”

image 18.jpg

**If outer boundary already existed, move it to bottom of edit list within definition of surface properties**

image 19.jpg

 For more Civil 3D information visit the Advanced Solutions Online Technology Center


Tagged with Surface, AutoCAD Civil 3D

About Leo Lavayen

Cary, NC Infrastructure Technical Consultant

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