Are Portolan Charts and Portolan Mile Geometrically Rooted in Classical Antiquity? A Cartometric Analysis of al-Shirazi's "Greek Map" and the Pisane, Lucca, Avignon, and Cortona Charts.

This article focuses on the cartometric analysis of some of the earliest known portolan charts—the anonymous Carte Pisane, Cortona, Avignon, and Lucca charts—and Qutb al-Din al-Shirazi's 1282 schematic described as a "Greek map." The study finds that the Mediterranean Sea was depicted nearly identically on the Carte Pisane and Lucca charts, with the Avignon chart fragment representing an intermediate stage between these and Pietro Vesconte's later charts. A notable discovery is that the square grids present on these charts and al-Shirazi's schematic share similar dimensions corresponding to 100 portolan miles (miglia) along the latitude φ = 36°, reflecting Ptolemy's underestimated Earth size and suggesting a misunderstanding of spherical versus Euclidean geometry by medieval cartographers. The research proposes that late medieval mapmakers may have combined classical spatial data—correctly scaled by Eratosthenes but longitudinally distorted by Ptolemy's smaller Earth model—resulting in the portolan charts' distinctive geometry, thereby supporting a hypothesis of pre-medieval, possibly classical antiquity, origins for their source material.