I got the lower-level-of-detail patches working. The scheme is something like this:
A bit hard to read (sorry!), but this was me working out how to navigate round all those triangles in an ordered fashion, to create a sensible triangle strip. Part-way through implementation looked like this:
Quite pretty in it's own right! Eventually I'd done it - it now looks like this:
You can see the "high-detail" joins. I'm also losing some of the rear-facing patches (though I don't do this right at the edges - we'll want to see any high "mountains" on those just-over-the-horizon bits).
Next steps - get it to dynamically choose either the high or low level of detail for each patch, and cull off-screen ones. Then I need to apply a height-map ... no idea how I'm going to get that to wrap properly yet ...!