Having worked on a couple of open world titles, I’ve had experience with two different teams methods of creating the basic surface mesh of a game wold. They both used at their core, a flat mesh that was deformed in a vertical axis by artists or imported height map data.
Texture mapping co-ordinates were projected along the same vertical axis as the mesh was deformed. This meant that the greater the slope of the mesh, the more distorted the texture appeared in the world, as is illustrated by the image below,
