There was a French guy, cartographer from XVIII century who was using very simple but incredibly laborious process to measure the curved France/ Europe. His adventures were described in the series "The worst jobs in history". It look like this. Climb the hill, climb another hill (with the equipment), make the triangle, measure the angles. Repeat, climb new hills. And guess what - it was roooound. This experiment could be done today, by any Flat Earther. Try, measure and prove in a month or less. If you like science and cars, you can follow me.
Funny that... there was a recent documentary made in Southern America , using state of the art equipment, telescopes lasers etc, the pieces of land that they were testing were indeed FLAT. Worth a watch. Think it was called convex earth the documentary. Many tests were done. Also here is a map found recently in a temple thousand years old, shows earth flat with more land outer Antarctica. Just how like Admiral Byrd of the US fleet mentioned in a live interview and in his death diaries.