Leveling is the process of determining the elevation of a point relative to another point. The curvature of the Earth and atmospheric refraction affect the result of leveling. There are techniques and formulas to correct the effect of Earth’s curvature and atmospheric refraction.
Flat-Earthers assume that construction works like roads, bridges, railways, etc. are built without accounting for Earth’s curvature. They are wrong. The leveling in such works are done in such a way it minimizes the errors due to Earth’s curvature and atmospheric refraction.