The Premedieval Origin of Portolan Charts: New Geodetic Evidence

Portolan charts are highly realistic medieval charts that show remarkably accurate coastlines of the Mediterranean and the Black Sea. They emerged suddenly, without any predecessors or a clear developmental path, in Italy during the thirteenth century. There is broad scholarly agreement that these charts are original creations of European medieval culture. However, corroborating evidence is lacking, and a convincing explanation of the method of their construction has so far not been provided. In this essay it is demonstrated by means of geodetic analysis that the overall shape of the coastlines corresponds closely to that on a modern map based on the Mercator projection. It is further demonstrated that this correspondence cannot possibly be due to chance. Consequently, the existence of a Mercator or Mercator-like map projection on portolan charts is incompatible with the assumed medieval origin of these charts.
Portolan charts are far more sophisticated than has hitherto been recognized. Their construction was well beyond the capabilities of cartographers from either mediëval Europe or the Arabic-Islamic world. This conclusion serves to reopen the question of the origins of the geometric data and the construction methods that until now have appeared to underlie medieval portolan charts.