No Horizontal Curvature
A sphere, despite what some may say, is a circle in three dimensions. No matter which way you look at you will always see it curve. Even if that curvature is very spread out, it will still be there.
So, let's take the supposed curvature in the round earth theory. To take one sample, as mentioned earlier, after 120 miles it dips by 1.8 miles. This applies as you look from left to right, in other words across, as opposed to when you look straight out. If anyone who has been on a plane or a high mountain knows, it is not difficult at all to see 120 miles across. Usually we see much more. And yet that drop of 1.8 miles is simply not visible.
There are those who claim that they have seen the curvature from planes. As this picture shows indicates, this is simply down to the curved windows on planes, which distort what you are seeing.
Views from mountains are less biased, and help drive this point home. This does require honesty on the part of the observer. 1.8 miles is a lot and would be visible. However, as is the case with false visionaries and other psychological conditions, if people want to see a curvature, they will see a curvature.