The Mercator projection was designed with certain uses in mind. 2 An accurate means to measure longitude at sea was only discovered in the second half of the 18th century with the development of the sextant and later the marine chronometer. However, 16th-century navigators used magnetic courses and not longitude and latitude values as Mercator’s map assumed. Theoretically this simplified oceanic navigation a ship captain could draw a straight line from one port to another, calculate the bearing, and maintain that bearing along the voyage. 1 Mercator’s projection had the benefit that straight lines drawn on the map are rhumb lines, lines of constant bearing that pass every degree of longitude at the same angle. To achieve his goals Mercator used a projection in which lines of longitude and latitude were made perpendicular at all values by increasing the distance between degrees of latitude as they reach the pole. As described in the third legend, Mercator aimed to maintain conformity to the shape of land masses even towards the poles and to have straight lines on the map accurately represent directionality. Mercator presented his map as not only an accurate representation of the known world, but also as a particularly useful map for the purposes of navigation. In 1569 the Flemish cartographer and mathematician Gerardus Mercator published a new world map under the title “New and more complete representation of the terrestrial globe properly adapted for use in navigation.” The title of the map points to Mercator’s main claim for its usefulness, which he expounded upon in the map’s legends.
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