Definition
Plane sailing solves the right-angled triangle formed by difference of latitude, departure and distance, using distance equals difference of latitude divided by cosine of course, and departure equals distance times sine of course. The flat-Earth assumption keeps errors negligible over distances up to roughly 600 nautical miles and away from very high latitudes. For longer runs, Mercator sailing with meridional parts replaces it, because the convergence of meridians can no longer be ignored.
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