So, this is a question I haven't seen brought up on this forum. Pilots of high-altitude airplanes can approximately tell which altitude they are at by flying horizontally and looking at the angle at which they see the horizon.

In that formula, 'r' is the radius of the Earth, around 6'000'000 meters, and 'h' is the elevation you are at in meters.

So, if you are at the Mount Everest, 9'000 meters above the sea level, the angle at which you see the horizon is 3.14 degrees, but that's probably not perceptible there because mountains hide the horizon. However, if you are at 30'000 meters, where some airplanes fly, it's 5.7 degrees, and that's perceptible. But how is that possible if the Earth is flat?