Terra forming is a bit of a pain because it's hard to see how flat an area is. It's worse with snow and when you add runing running with low graphics settings...

What would be idea is a small grid or show the polly lines for the area that would be effected if you did a terrain action. Handy for controlling what you effect. And for survaying what you want to do, have extra grid area around the effect area but in a different color/brightness. Make the boarder area the same width as the effect area.

Maybe allow putting temp stakes in the ground also for marking off an area, but unlike with other objects, these would not prevent you from terra forming or planning a building/wall. Once you are done working on the area, you could then pull up the stakes for use else where.