I've also added a toggle with an additional atlas to show the terrain edges. Here is an example.
|Some basic terrain edges|
|Terrain edges that are obviously different for visualization|
I'm very happy with how this has turned out. It would be a fairly solid foundation for a procedurally generated overhead map system. There are some things I'd prefer worked better (the GID less tied to the tile types for example, or hills and mountains being better for edges), but overall this is a good starting place.
You can get the code here.