A medieval map mystery may be solved

Portolan charts - nautical maps of Mediterranean and nearby coasts - appear in Italy around 1270 already remarkably accurate, with no surviving sequence showing how their precision developed. The standard explanation, that generations of sailors’ compass bearings and estimated distances were combined, cannot account for their accuracy: simulations of that method produce maps roughly ten times less precise. Geography professor Tome Marelić has instead used computer-assisted cartometric analysis to compare chart locations with modern coordinates, measuring both overall errors and the patterns behind them.

His findings suggest that the charts were assembled from older maps rather than built from medieval navigation data alone. When a chart is divided into regions and each region fitted separately to modern geography, accuracy roughly doubles. Some sections on the earliest surviving charts closely match those on later maps by Genoese cartographer Pietro Vesconte, and comparable coastlines remain geometrically unchanged on charts produced centuries later. These patterns point to inherited map material. Marelić’s latest study proposes that the underlying data may date to classical antiquity, and that a map projection associated with the first-century geographer Marinus of Tyre may help explain the charts’ geometry. Historical records and quantitative measurements support this account, including a possible reconstruction of how Vesconte combined such material to create a chart.