Constant heading line (loxodrome)

A constant-heading line, the loxodrome or rhumb line, is the track a ship follows when it holds a single true course, crossing every meridian at the same angle. It is the natural line to steer because the helmsman keeps one heading, and it plots straight on a Mercator chart so course and distance come straight from Mercator or plane sailing. Against the great circle it is longer except on the equator or a meridian, and the higher the latitude and the longer the leg, the larger the penalty. Long ocean passages therefore approximate the great circle as a chain of rhumb-line legs between gnomonic waypoints.