Ref the shape of the earth question - the practical application of navigation considers the earth a perfect sphere for journeys up to 400' and an oblate spheriod for distances beyond 400'.
In fact up to 400' you can consider the earth as flat (plane sailing). After that distance we get into the relm of mercator sailing and (god forbid) the PZX triangle to calculate great circle waypoints using spherical trig.
Thank god for the FMS!!