Likewise, flying in a plane for any distance and seeing the course is identical to if you pulled a string tight against a globe (give or take). I suppose pilots could choose to fly in an arch to keep the secret, but eventually some capitalist has got to spill the beans to cut costs, right?
Also: the southern hemisphere.
It doesn’t really matter if the earth is flat or not, for them it’s about being part of a community of people who believe in something that society tells them is wrong. There is no changing minds besides the 1 in a million you hear about and so all the calculations you could do would never work, sadly.