I have access to some pretty remarkable propagation simulation software, at work. It's used to give good predictions of coverage for indoor, outdoor, and mixed RF systems, during design. When fed with good terrain data and "clutter" data, as well as accurate data describing the RF equipment at both ends of a system, the results are shockingly accurate.
Terrain data describes the surface of the earth's elevation above sea-level, just like a traditional topo map, but far more accurate. I used both data from the space shuttle "SRTM" data and USGS digital elevation map "DEM" data for these examples. "Clutter" data describes the environment above the surface and how far up it extends. By way of example, there are 6 different attenuation values for "forest." Clutter data is "for pay" data and I won't have access to it for our area going forward (but if I wanted to repeat the simulation for Kigali Rwanda, I'm set!).
As the Imgur album says, I produced what I believe are good predictions for 2-way coverage with a person standing at ground level and a typical HT and stock antenna. Higher gain antenna and elevation extend this footprint pretty significantly.
If you have interest in a close up of your area or a simulation of coverage with your antenna (higher gain or elevation) setup, just ask! It'll be very easy for me to run these through Friday and will only take a day or two, after that.