The Mercator Projection and Its Distortions

The Mercator projection was devised in 1569 by the Flemish cartographer Gerardus Mercator (born Gerard de Kremer, 1512-1594) for his world map "Nova et Aucta Orbis Terrae Descriptio ad Usum Navigantium Emendate" ("New and Enlarged Description of the Earth with Corrections for the Use of Navigators"). Its defining mathematical property is conformality: it preserves local angles and shapes at every point, which means a straight line drawn anywhere on the map corresponds to a rhumb line, or loxodrome, a line of constant compass bearing on the actual sphere. A navigator could therefore plot a course between two ports, draw one straight line on a Mercator chart, and sail that single fixed heading the entire voyage without recalculating direction, an enormous practical advantage in the era of magnetic-compass sail navigation. Mercator achieved this by progressively stretching the spacing between parallels of latitude as they approach the poles, in exact proportion to the east-west stretching already imposed by drawing meridians as parallel vertical lines rather than lines converging toward the poles as they do on the actual globe.