The origin problem of nautical cartography: the importance of evidence and method

The origin of nautical or portolan charts poses the greatest challenge of the history of cartography. Claims that the 'big' questions have been answered and research can therefore focus on filling in the details are unfounded. Majority opinion still holds that portolan charts are medieval creations. Although there is a complete absence of historical evidence that supports a medieval origin, evidence that contradicts such an origin does exist. In the absence of historical records, quantitative (cartometric) analysis is a relatively novel technique that can reveal new facts about these charts. Cartometric analysis has shown that portolan charts are composites of accurate regional charts. These regional charts agree surprisingly well with a modern map on the Mercator projection. Both their correspondence with this projection and their accuracy are incompatible with a medieval origin. This has revealed a conundrum that is still able to raise the temperature of any debate on this fascinating subject.