In the Flight Simulator scenery system, an area within a 40km radius (a square area of 80 x 80 km) is read into the seed map and projected as ground texture (as part of the graphics database). When textured ground is active, the horizon system doesn't generate any ground; instead, the horizon system relies on the textures from the graphics database to cover the ground. In these cases, the area beyond 40km isn't generated to the horizon edge by the textures from the seed map. The horizon trapezoids system fills in this area.