MapEd: how to flatten a bent terrain?

[RootlessAgrarian]

Having built my first bridge (from the excellent PM YT tutorial) I got ambitious and tried to introduce a lake into my beginner map exercise

I created a low point in the ground, made a Terrain with a water-lake texture, slid it into place, and started editing to make it pretty. Turned on reflections and added some trees and rocks to camouflage any rough spots. It was looking not so bad, but then I noticed something distressing.

Without knowing how I did it, at some point I bent my lake surface! I can see when I fly around it that it's deformed, no longer flat... oops.

Is there any good way to flatten a bent terrain? or should I just delete it and start over?

is there any way to tell the editor that a terrain (a water plane for example) has to stay flat and should never bend or even tilt?

BTW, why do terrains have only 2 handles? I would have expected one at each corner, and yet there are only 2 on one edge.

[ETS2-User]

Set noise power to 0. For better flexibility use bezier patches instead of terrain.

[RootlessAgrarian]

The answer to my last Q has occurred to me at last. Would I be right in believing that the Terrain is just a road segment without the paving feature, so it has the same 2 handles as a road segment?

[ETS2-User]

Yes, which makes it practical for bridge heads as you can just snap it to the road's terrain. But other than that, bezier patches are more practical.

[RootlessAgrarian]

Aha! I noticed the use of terrains in the PM video, to make attractive natural-looking banks at bridge heads: and now it makes sense. When I snap one road segment to another, their respective terrains try to mesh gracefully (if they can!). And that's just what the terrain does when I snap it to a bridge head node. OK, understood.

So in general, one should use terrains for bridge heads and flat bodies of water, but use bezier patches for filling in awkward spots elsewhere? I had read that bezier patches are expensive to render so one should not use too many. Is one large bezier patch cheaper to render at runtime than several small ones?

[ETS2-User]

That's definitely true. You can also set the weight on rendering with the X and Y tesselation values in the item properties. It's therefore also the tool for setting the size of the stamp grid.