Distance-Drop over the earth
A (very simplified) model of the ellipsoid showing anticipated vertical drop over a given observation with a given earth radius.
A nominal radius of the earth is 6371000m, more precise models can be calculated from the information here: http://home.online.no/~sigurdhu/WGS84_Eng.html
Drag the right hand point left and right to adjust the observation distance, and get an estimate of the difference between a straight line and the surface of the earth.