Mathieu,
How much surface can I fit into my 12 GB WIN 7 64 machine? The DEM grid spacing is 10 meters. That is already pretty coarse so I wouldn't want to thin it any more. In Civil 3D we can turn off surface display to reduce memory resources. Even though the surface is not displayed it can still be queried and sampled for design purposes. Does OpenRoads work similarly? What I am thinking is I could store the DEM in a separate file and reference it as needed for my study. If I keep it's display turned off in my host file how will the performance compare?