Ptolemy's Triangulation of the Eastern Mediterranean

Abstract

C OMPOSED ABOUT 150 A.D., Ptolemy's Geography remained for centuries the principal model for cartographers. The work codifies the data required for a map of the known world, determining the contours of coasts and mountains and listing names of cities, rivers, and peoples. I shall treat this work as a map. As for the physical contours, the influence of the Geography is evident only with regard to certain kinds of detail, primarily errors, one of which is the subject of the present paper-the false orientation of the Palestinian coast. With regard to the names, Ptolemy ranks with Strabo and Pliny as the most reliable compiler of authentic information. The false contours of Ptolemy's map reappear very rarely on the thirty ancient maps of Anatolia that I have examined: the false orientation of the Palestinian coast marks several Arabic presentations of the ninth century, and plate 26 of Johann David Koeler's Descriptio orbis antiqui (Nuremberg, 1720) shows all the aberrant features of Ptolemy's Anatolia, coast lines, mountains, choice of names. The map of 1515 by Ubelin and Essler ' owes much to Ptolemy, as does the Carte generale de l'Empire du Turc of 1646. Mercator's map of 1569, at the very small scale of 1 :14 M, reveals nothing precise regarding contours and names only sixty Anatolian places, a few in classical spellings and possibly from Ptolemy (Apamia, Nicomedia, Laodicia), many more in Italo-Byzantine guise (Carpi for Calpe/Kirpeh, Trebisonda for Trapezfis/Trabzon, Aiazza for Aigaiai/Ayas), and several with the prefix eis of Istanbul (Simiso for Amisos / Samsun, Isnich for Nicaia / Iznik). Mercator had pertinent but fragmentary information on places unknown to the classics. The Geography consists of a long preface, occasional short remarks in prose, and massive lists of place names, carefully grouped under various subheadings and situated by their coordinates expressed in multiples of 5'. The work is usually quoted for the names, as if they were set more or less in correct geographical relationship. However, to profit further from the work and to identify thousands of unknown places, one must examine the coordinates in the light of Ptolemy's method of triangulation, hoping to find the reasons for the apparent numerical errors in the case of well-known places. For example,