Editing Mercator projection (section)

===Marine navigation===
The Mercator projection was designed for use in marine [[navigation]] because of its unique property of representing any course of constant [[bearing (navigation)|bearing]] as a straight segment. Such a course, known as a [[rhumb line|rhumb]] (alternately called a rhumb line or loxodrome) is preferred in marine navigation because ships can sail in a constant compass direction. This reduces the difficult, error-prone course corrections that otherwise would be necessary when sailing a different course.

For small distances (compared to the radius of Earth), the difference between the rhumb and the [[great circle]] course is negligible. Even for longer distances, the simplicity of the constant bearing makes it attractive. As observed by Mercator, on such a course, the ship would not arrive by the shortest route, but it will surely arrive. Sailing a rhumb meant that all that the sailors had to do was keep a constant course as long as they knew where they were when they started, where they intended to be when they finished, and had a map in Mercator projection that correctly showed those two coordinates.{{sfn|Osborne|2013|pp=39–40}}