Mapping North America

This portolan-style nautical chart was probably drawn in Genoa, c. 1320-1350. It covers most of the Mediterranean Sea and also covers the western part of the Black Sea. These maps were useful for navigation in relatively small bodies of water, such as the Mediterranean, Black or Red Seas. The makers of portolan maps could not accurately scale their charts or project three-dimensional models onto a flat surface, however, so they were of no use to those wishing to cross the open ocean.

As with his model of the solar system, Ptolemy assembled geographical information into a grand scheme. He created a grid that spanned the known parts of the globe, assigning coordinates to every place and geographic feature. Latitude was measured from the equator, as it is today, but rather than using degrees of arc Ptolemy recorded the length of the longest day at the given latitude. He put the meridian of 0° longitude at the Canary Islands, the westernmost land that he was aware of.

The uses of longitudinal and latitudinal lines and the specifying of terrestrial locations by celestial observations were perhaps the most significant contribution of Ptolemy’s maps. When Geographia was translated into Latin at the beginning of the fifteenth century the idea of a global coordinate system revolutionized geographical thinking, putting it upon a scientific and numerical basis.

The top image shows a map reconstituted from Ptolemy’s text sometime in the 15th century. The second image shows a version published in 1482 by the German cartographer Nicolaus Germanus as part of his Cosmographia. This was the first atlas to be printed in Germany, as well as the first made from woodcut blocks and to contain hand-colored maps.

As drawn by these cartographers, the map distinguishes two large seas that are entirely enclosed by land, the first one being the Mediterranean, the second one being the Indian Ocean (Indicum Pelagus), which extends into the China Sea (Magnus Sinus) in the East. It is questionable whether Ptolemy would have actually drawn the features at the map’s extremes, as scientific principles would have argued against drawing what he had not experienced physically or heard from eyewitnesses. Major cities like Alexandria, Rome, Athens and Jerusalem are not indicated on the map, probably because it was only meant as a general reference guide to the individual regional maps which followed.

Ptolemy’s text and the maps derived from it set the pattern for modern cartography. Henceforth, world maps would be based on a grid or mathematical projection, usually with north at the top and east to the right. The distance between the equator and each pole would be 90 degrees of latitude with the equator being zero. Ptolemy’s world map is thus the mother map of modern cartography and an essential component in the discovery, exploration and mapping of the New World.