Abstract

In his Geography, Ptolemy recorded the geographical coordinates of more than 6,300 toponyms of the known oikoumenē. This study presents the type of geographical information that was used by Ptolemy as well as the methods he applied to derive his geographical coordinates. A new methodological approach was developed in order to analyse the characteristic deviations of Ptolemy’s data from their reconstructed reference locations. The clusters of displacement vectors establish that Ptolemy did not obtain his coordinates from astronomical observations at each geographical location. The characteristic displacement vectors reveal how Ptolemy derived the coordinates: he constructed locations on maps using a compass and ruler, for which he employed a small amount of astronomical reference data and geographical distance information; he made schematic drawings of coastlines, based on textual descriptions of coastal formations; and he situated additional locations within the established framework using reports of travel itineraries.