You can't, because terrains add and remove triangles on the fly to optimize poly count for each frame.
This optimization process is actually fairly slow, and can take quite a few precious milliseconds of time each frame. I've found that I'm always better off finding a different way to do landscapes.
This optimization process is actually fairly slow, and can take quite a few precious milliseconds of time each frame. I've found that I'm always better off finding a different way to do landscapes.