Agree with Mukka.
If you draw a tangent at the earth's surface, and travel along it 60nm, you will find there is approximately a 0.5nm 'drop'.
R = radius of Earth (nm)
Tangent is 60nm long
Therfore for a right angled triangle,
Hypoteneuse (H) = root(R^2 + 60^2)
The 'drop' is H - R = ~0.5nm.
Regards
Fat Dog