When I originally build the backdrop for the town of Hills, I only built what I assumed I needed for the scene. The plan was that the Eldon C. Stutsman structure would block most of the horizon, eliminating a lot of wall space that needed to be covered.
When I actually built the Stutsman complex, it more than covered the horizon horizontally. Vertically was a different story.
The small warehouse at the north side of the complex was, in fact, too small to cover the edge. Once everything was in place there was a very apparent gap in coverage.
A quick jaunt through Google StreetView found an appropriate image to fill the space. Once printed and applied to the backdrop, the gap was closed, extending the horizon for miles and miles.
The Hills Line 