Perhaps this has already been posted here. (?)
https://en.wikipedia.org/wiki/Salar_de_Uyuni#/media/File:Salar_de_Uyuni_Décembre_2007_-_Centre_de_Nulle_Part.jpg
Go to the wide photo at the bottom of the page.
a) Copy the photo to your computer and enlarge it to fill your screen from top to bottom.
b) Put your thumb nail on the horizon.
c) Move the photo from the left, and all the way to the right.
— There is no curvature.
— There is no peak in the center.
(NOTE: You will notice that the left side is a little higher. This is because the photographer didn't level the shot perfectly. You'll also notice that right over his vehicle there's an anomaly where the horizon fails to meet up perfectly. The difference in height at this point makes up for the height difference at the left side of the shot.)
Experiment: With a 50mm lens - Go to the ocean and take two pictures of the horizon, one photo from your LEFT (photo #1), and one from your RIGHT (photo #2). It doesn't matter if the shots perfectly overlap, because "earth curve" will be noticeable in any shot, if it's there.
Take photo #1 and overlap it so that the right side overlaps the center of the photo #2.
If the earth is round, the right side of photo #1 will dip into the middle of photo #2, as the center of each photo should be higher than at the edges.
If you reverse the photos you will have the same results.
If it does not dip into the center of the other photo, then there is no curvature.
(A 50mm lens is considered to be the way we see things—it's TRUE. Anything below 50mm is a "wide angle lens," and will
distort the photo. Photos that make earth appear to have a curve are taken with wide angle lenses, likely around the 30mm range. If you are a photographer and have a large format camera this experiment will yield better results.)
—Rx