I think if there are any flat earthers that really and truly believe that the earth is flat, they should either.:
(1) Join the Navy and see the real, round earth
(2) Go on a cruise ship and see the real, round earth
(3) Go down to the sea shore and watch ships come and go over the horizon on the real, round earth
The earth is a real, round earth.
If you have ever worked in any of these fields, in particular with those dealing with the horizon, you certainly have to deal with the curvature of the real, round earth. :
(1) In the Navy, lookouts estimating the distance to the horizon
(2) In certain types of surface search radars estimating the maximum range by the distance to the horizon
(3) In certain types of microwave repeater systems, for estimating the optimum spacing between repeater stations by the distance to the horizon.