Ptolemy's map and ancient Periploi

Ptolemy's map is for the first time compared with the data on the length of the coastlines transmitted by ancient periploi. As a result, many close coincidences are traced between them, showing that Ptolemy's map was much more heavily based on these or similar peripli than it has been assumed until now. Additionally, the influence of Ptolemy's underestimated value for the circumference of the Earth on his map is revealed. Namely, two interrelated distortions are observed to result from this error. First, coasts oriented meridionally were compressed in comparison with the distances recorded in periploi. Second, this compression was compensated by proportional stretching of adjacent coasts oriented latitudinally. In particular, these findings suggest a simple explanation for the strange shape of the Caspian Sea on Ptolemy's map.