Line Of Sight Routing
If we deal with a perfect sphere, the direction doesn´t matter, however the height of the source does. Assuming a minimum height of 10m, the line of sight distance and altitude can be calculated. If you want more information for a radio link you will not do better than starting of from the propagation button.
How far can you see?
Remarkably (on a sunny day that is), due to that fusion reactor in the sky (read sun) you will be able to see further than where most radio waves will reach.