Totally true. More in general, it is a relatively simple mathematical fact that a sphere without a point is homeomorphic (even conformally!) to a plane. Therefo

Totally true. More in general, it is a relatively simple mathematical fact that a sphere without a point is homeomorphic (even conformally!) to a plane. Therefore, it is impossible to prove that the Earth is spherical unless you either do measurements and run numbers (like you propose) or visit every single location of it (including all the seas and both poles; particularly the south pole, which for some reason flat earthers tend to assume it is missing). The only way to do the latter, as far as I can tell, is to be in a sufficiently polar orbit for enough time (or at least have a satellite in orbit), and few people are able to do that (directly, i.e., without trusting somebody else). It is interesting that is not uncommon for flat earthers to be more "scientific" that many spherical earthers in recognizing that many spherical Earth arguments are not sufficient by themselves. Of course the spherical earthers happen to be right, while the flat earthers then proceed to invent completely implausible theories to support their beliefs, but they deserve some recognition for the first stage.