I'm trying to calculate the heights of birds from a plane flying at a known altitude above them using the parallax effect. They're being recorded from the plane and the head and tail end coordinates marked on each frame they appear. The equation I'm using is from here: http://www.geog.ucsb.edu/~jeff/115a/lectures/stereoscopy_and_height_measurement.html specifically this part: http://www.geog.ucsb.edu/~jeff/115a/lectures/stereoscopy_and_height/displacement.jpg h = H*(dP/(P+dP)) Which I think holds up if the object you're measuring is horizontal as well as vertical, it just produces two triangles to compare (in the case of birds at the head and the tail) instead of one. Am I right about that? The problem is the heights I'm calculating are a about 200m above what I'd expect. The speed of the birds is also a factor but I have a way to attempt to compensate for it. That's what I'll be looking into next if this height equation isn't correct in this case. Any insight would be welcome.