A few points:
1.) This was experimentally tested in Norfolk England, first resulting in a finding that the earth was flat (!). More careful measurements later proved the earth convex (whew!) The effect of atmospheric refraction on the results has been noted:
2.) Cyrus Teed (“Koresh”) and his Koershans backed a similar experiment by Ulysses Grant Morrow in 1896 along the Illinois Drainage Canal, which proved the earth concave, just as Teed believed. Critics blamed atmospheric refraction. (Same cite. see also Martin Gardner’s Fads and Fallcies in the Name of Science. Alsi The Big Book of Weirdos)
3.) In Hal Clements science fiction classic Mission of Gravity, the Mesklinites, living on a high-gravity planet, are convinced that they live in a bowl-shaped environment. They know it isn’t flat, because they’ve worked out the geometry. But it looks like a concave surface to them, because in their case, atmospheric refraction is severe.