To prove curvature, observe sunset from sea level. Wait until the sun has completely disappeared and then the moment it does, elevate your position by a few meters. The sun will again be visible, proving earth's curvature. To do this on paper, draw a circle then plot a straight line from a point on the circle such that it is at right angles with a diameter line. This is your viewpoint at sea level. Then draw another line, originating a few mm above the originating point of the first line and draw the line such that some point along this line connects with the curvature of the circle (second line travelling in same direction as first but angles downward. The space between the two lines is the difference in field of view due to elevation. This proves curvature. If the world were flat, we would be unable to observe the sun setting like this. Go try this for yourself and see that the spherical earth is real compared to an impossible flat earth.