Or, simply do what I do! You see, there's no need to fix a problem if no one can see it anyhow. Have a problem with edges? Hide the edges! Here's how it all plays out. Your terrain looks something like in shot number one, with edges flared out. Problem is(as in shot 2) you can see strait under it, which makes a big ol' mess of things. One way to fix this is simply take the edges out of your viewing. Using the Camera controls, zoom in close to the terrain. I expand the thing, making it considerably larger, till the edges just float out of your field of view, but the big(or small) dunes or whatever they may be are still all over the place. Also make sure you cover the Horizon line with the Terrain, or something else, otherwise it wont look quite right. But, of course, its all up to you, so if you think it works, go for it. Thats about it, and I hope I've solved your problem.