Mark received this question from a long-time InRoads user in Nevada: 

Dear Obi-wan, does InRoads have a surface editing command similar to the old Autodesk LDD 'flip face' tool? You know, sometimes that can be very handy.

Thanks much!

- Anakin

Here is the Zen Dude's response:

As a matter of fact, yes, InRoads has that functionality, although it's fairly new.  And the InRoads era before this tool was added had always been a sore spot with Autodesk users moving to InRoads.

It's called Change Triangle Edge and can be found under the Surface > Edit Surface tools, and is a pretty straight-forward command.

First let me describe what is occurring with the Change Triangle Edge tool. This discussion will cover the 'mechanical' or 'programmatic' aspects of this tool and exactly what is occurring when this tool is used; it won't really cover 'why' you would use this tool.

Every 2 adjacent triangles have a single common leg. That common leg is the leg that is 'changing' when this tool is used. But it's not just changing a single triangle leg, it's changing 2 legs; "flipping" or affecting 2 triangle faces, not just one. To add to this, remember, it's only affecting 2 triangles, not 3 or more (keep this in mind as this explanation continues).

Look at the picture shown here and pick any one green triangle leg.

That leg is part of two triangles in every case (unless it's on the outer perimeter of the surface). Those two adjacent triangles will be affected if that leg 'flips'.

Now focus on a common leg and the two triangles that share that leg. There will be 4 corners (indicated by the purple dots) that bound those two triangles that you've chosen.

After you've located a common leg, and its 2 triangles, and its 4 corners, imagine changing the common leg (connected to two of the corners) to the other two corners.

This is what the Change Triangle Edge tool does.

Look at the examples shown here where I've drawn an orange line to the other two corners of the two triangles sharing a leg.

In doing this little mental exercise you may have observed a potential problem. If you haven't caught on to that yet, it will be brought up in more detail now.

The Change Triangle Edge tool works great is some instances, but not in allinstances.

In order to find the instance where this tool will produce undesirable results you have to look beyond just the leg to be changed and look at the quadrilateral (4-sided) shape formed by the 2 triangles sharing the leg to be flipped.

In particular, this tool has a problem when the overall shape formed by the two triangles that share the leg (about to be flipped) contain a vertex that is equal to, or greater than 180 degrees.

Specifically, the problem occurs when the current triangle leg is connected to a vertex that forms the central point of an angle greater than 180 degrees.

If you do the same mental exercise that was done earlier you'll be able to envision how the leg will inappropriately form if it were to be 'flipped'. And it will reconnect this way in InRoads even though it seems to violate all the laws of DTM construction. 

On top of that, I can pretty much guarantee you that the resulting contours will not be to your liking ; )

So, unless you know exactly where a 'flip' like this is leading you, it is recommended that you avoid these types of leg changes.

TIP #1: Visualize the 'flip' before you do it. If the flip goes sour like shown above, (there isn't the usual "Accept / Reject" option when executing this command) InRoads can't seem to relocate that leg and hence has an inability to 're-flip' it back to its original position. ...and there isn't an undo on this tool either. The only other alternative is to re-triangulate the entire surface.

TIP #2: Basically, don't flip a triangle leg if the new leg position forces it out of the quadrilateral area formed by the two triangles that share the common leg (unless you plan to continue flipping other triangle legs to remedy any overlapping legs).

TIP #3: Adding a breakline to control a triangle leg is a more stable method of controlling triangulation and has been the historical method that most InRoads users have employed to address the need to 'flip' a triangle leg.       

I hope this helps.

Civilly yours,  

- zen