Well, if you're looking for ideas, I have an idea to work with (that could potentially be wrong :P). And since I'm bored and have way too much time on my hands, I made an image to illustrate
30 yards would be a measured distance (how? cast range maybe?) and at each end you get the coordinate. In this case, you get a distance of 30 yards between (30,40) and (30,45), meaning there was a change in 10% in the x-axis and a chance of 5% in the y-axis.
Oh, and my math could be completely wrong... hopefully it sparks an idea though!