Art and science of representing a geographic area graphically, usually by means of a map or chart. Political, cultural, or other nongeographic features may be superimposed. Ptolemy's eight-volume Geography showed a flat, disc-shaped projection of part of the Earth. Medieval European maps followed Ptolemy's guide but placed east at the top of the map. In the 14th century more-accurate maps were developed for use in navigation. The first surviving globe dates from 1492. Discovery of the New World led to new techniques in cartography, notably projection of a curved surface onto a flat surface. In particular, Gerardus Mercator projected landmasses onto a cylinder wrapped around the Earth's Equator. Such cylindrical projections maintain proper directions or bearings, though they cause distortions in distances at high latitudes. Contour maps show relief by connecting points of equal elevation with lines, mean sea level being the reference point. Modern cartography uses aerial photography and satellite radar for a degree of accuracy previously unattainable. Satellites have also made possible the mapping of features of the Moon and of several planets and their moons. See alsogeographic information system; global positioning system.