Airplane pilots or folks who have circumnavigated the globe by sea would be unable to observe any curvature. Some have not succombed to Heliocentric indoctrination. It is often admitted by Heliocentrists that the curvature of the Earth cannot be seen from a plane. Certainly, those who travel by sea have some of the best evidence against Heliocentricity, as lighthouses can be seen beyond where the horizon should be on a ball-Earth (i.e. over the alleged curve of the horizon). Next time you are on a plane, check out of both windows. On a clear day, you will be able to see the horizon through both at the same time - the horizon rises to the eye level. This is because Earth is flat - on a ball Earth, the horizon would/should drop away the further from the plane the land was. However, according to some die-hard Heliocentrists, this is simply because a plane is not at a high enough altitude to observe curvature.
As stated above, the Earth's curvature cannot be seen from a plane, and certainly not a ship. Some die-hard Heliocentrists argue this is simply because a plane is not at a high enough altitude to observe curvature. NASA is required by Heliocentrists, because it is the only organisation to have claimed to photograph ball-Earth. However, their evidence is fraudulent.
In Heliocentricity, the gyroscope should have flipped by the time a plane crosses from the Northern Hemisphere to the Southern Hemisphere. That it doesn't is proof that the Earth is flat.
No they haven't. Just because someone thinks he has seen something doesn't mean he has. The evolutionary archeologist may think he has seen evolution in fossils. He hasn't. Heliocentrists think they see the ships travelling over the curve of the horizon. That a telescope will bring the ship back into view proves their belief wrong.