Rhumb Line

A path on the Earth’s surface that crosses every meridian at the same angle.

A rhumb line represents a constant true course on an idealised surface and plots as a straight line on a Mercator chart. Except along a meridian or the Equator, it is generally longer than the corresponding great-circle route between two points.

On long passages, high latitudes and non-Mercator charts, projection and geodetic effects matter. Electronic systems may offer rhumb-line or great-circle route calculations, so the selected mode and any waypoint approximations should be confirmed rather than inferred from a line on the display.