Are Portolan Charts and Portolan Mile Geometrically Rooted in Classical Antiquity? A Cartometric Analysis of al-Shirazi’s “Greek Map” and the <i>Pisane, Lucca, Avignon</i>, and <i>Cortona</i> Charts

Some of the earliest-made known portolan charts ( Pisane, Cortona, Avignon, and Lucca charts) and Qutb al-Din al-Shirazi’s schematic (1282) resembling “a map of the Mediterranean that was drawn by the sages of Greece and the ancient geometers” were cartometrically analysed. The results show that the Mediterranean was drawn nearly identically on the Carte Pisane and Lucca chart and that the preserved fragment of the Avignon chart is somewhat of an intermediate step between the Pisane–Lucca model and Pietro Vesconte’s model. The coasts of al-Shirazi’s schematic fit well with the Pisane and Cortona charts, whereas its 40×30 grid is similar in size to the grid drawn on the Pisane and Avignon charts. The length of its sides along the latitude φ = 36° (125 km) corresponds to 100 portolan miles ( miglia) and a 2-degree interval along the parallel φ = 36° according to Ptolemy’s incorrect estimation of the Earth’s size. Hypothetically, such a misunderstanding could have occurred if a late medieval cartographer (unfamiliar with the differences between spherical and Euclidean geometry) erroneously combined different spatial datasets from classical antiquity by unwittingly superimposing Ptolemy’s longitudes onto maps or charts made according to Eratosthenes’s correctly obtained size of the Earth by treating them as distances.