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Another method would be to climb a hill of at least 500 feet overlooking the sea, take a photo of a nice clear horizon, and then when you get home stretch it vertically. Straight lines stay straight, while curved lines are exaggerated, like this:
Note: when taking photos like this, great care must be taken to ensure that barrel distortion is not creating or exaggerating the curve of the horizon. There are different ways this can be done.
pared with what these views would look like on a flat earth.
So about barrel distortion, it curves straight lines around the center of the picture.
So you'd want to take two photos - one with the horizon slightly above the center of the picture, and the other with it slightly below.
Stretch them both equally, and take note - if they both curve the same way, then it's curve of the horizon.
However, if one curves up and the other curves down, then you're mostly seeing barrel distortion.
In reality, even with a curved earth if such were the case, you would probably see a combination of curve plus barrel disortion, so the photo with the horizon above center would show more curve and the other would show less curve or straight, or a negative curve, but not as much of a curve as the above-center horizon photo.
That's what's tricky about gopro fighter jet flight videos - even the default lenses have some barrel distortion and you can see that the horizon bulges up if it's above center, and sags down (concave) when it's below center of picture. You'd have to freeze frame the video right when the horizon line was in center of the picture, and see if it showed curve at that point. But even then, many early gopros had rolling shutter which can create rolling wave patterns in the picture so even if you saw a curve in the center of the picture it could be from the vibration of the jet and the rolling shutter.
What you'd really want to do is go up to a high point, and stretch a thin black string super tight between two trees. Maybe tie a couple helium balloons to it for good measure so it can't sag down.
Then step back move your camera so the string lines up with the horizon, and take a picture. That way you're comparing the horizon to a known straight line before it enters the camera's optics and gets distorted. That way, if the optics curves the horizon, it'll also curve the string equally.
If that showed a curve, ok now you'd have my attention. Just make sure the string is super tight, because if it sags down it'll make the earth look like it's curved up.
That's why the helium balloons to guarantee that if the string is curved at all, it's up. Then if the earth curves above that, we know we got something. But don't worry, nobody's tried this yet. When they do, we can see that it's as flat as a pancake. Flatter, actually. I plan to try it myself when the snow melts in the mountains.