close enough
500'
At least that's how I constructed it in AutoCad. See my reply to ER for why the error came out in the trig.
There are a couple of different approaches. I tend to do these things in CAD, because I can do it there without much thought. I like to double check the trig, just to convince myself I haven't flipped something.
I used sine law to get the bases of the observer's vertical triangles from the planer triangle and the rest is just pythagoras.