I'd like to suggest adding a marking that represents the visible distance to the horizon from the point of origin (assuming perfect weather conditions and an aproximated earth curvature). The purpose of this is to figure out at any given point, where exactly will the sun rise and fall.
The calculations for this are fairly simple and are described at http://blogs.discovermagazine.com/badastronomy/2009/01/15/how-far-away-is-the-horizon/ . I believe elevation information is available from the map, although it'd be nice if I could specify it manually as well.
Regardless, this tool is absolutely fantastic. I will refer to it every time I want to plan some photos. Спасибо!