First - do you believe that (for example) Spain exists? Why? You didn't see it. Maybe photos of Rome is just a fake?

Now for real. Distance from A and B (for example 400km). For car it will be... 400km. But if you believe in GPS - measure this distance in plane. It will be much longer. Why? If earth is flat then distance should be nearly equal. Why it doesn't? Because plane high - which is just radius of CIRCLE - is longer. Simple proof of curvation of earth

If noone land on the moon why there are mirrors? http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment

If you are really believe it - proove it by REAL evidences (f.e. build jet drone, which can reach stratosphere and make photos of _curvation flat earth)