This question already has an answer here:
EDIT: OK, I have been asked to clarify my question(s).
At what altitude do we start to see the curvature of the Earth?
Distance to the horizon at this altitude.
The angle between line of sight of the observer and vertical.
My reasons for these questions are:
1) I have read pilots' comments saying they cannot discern a curve at 40,000 feet. I was surprised because I have been on open sea and believed I could see the curve at the periphery of my vision. I thought about it and realised if every point on the horizon is equidistant to me, then I would probably have to get really high to see the curve. So maybe my brain created the curve, as peripheral vision is largely deduced rather than 'seen'?
2) Flat Earthers are bugging me.