Yeah, dowsing will often aim at far off stuff through walls to troll you.
Basically, if you want to make it easy, aim it so it's "in line" (aiming in the right direction, if you move it off slightly and arrow will tell you to point it back), then pull up your map, and make a mental line straight forward from where you are looking. Then move like 20 yards left or right, lock on your dowsing again, pull up map again, and figure out what is in a line with where you're looking now. Where your mental lines cross is where you'll find your target.
I found that useful at least, because I didn't have to dick around thinking it was right near me when it was clearly going to be on the other side of a mountain or something.
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