I've been piloting a drone yesterday and made a video hovering at around 100m, the visibility was very good. I've shown the video to a friend of mine and we were arguing whether it's the curvature of the Earth visible at horizon, or is it just camera lens disturbing the image.
I'm rather sure it's curvature of the Earth, but still don't know how to prove it. I already checked that on the lower part of the picture the buildings-roofs are lined up straight, no disturbance. Yet the horizon is slightly bent (checked by putting a piece of paper on the screen). Which I think already proves the point: should lens disturb the picture, the disturbance would be visible both on the upper and lower part of the picture symmetricly.
I've read How high must one be for the curvature of the earth to be visible to the eye?, but it doesn't help me just yet. As far as I understand, the above link help calculating the distance to the horizon depending on the altitude. But why is the horizon bent?
I would appreciate anyone helping me straighten my thinking :).