Portolan charts, it was always assumed, were compiled by medieval European mapmakers from contemporary sources. A Dutch doctoral dissertation now disproves this: these nautical charts are impossibly accurate, not just for medieval Europe, also for other likely sources, the Byzantines and the Arabs. So who made them – and when?
Mystery has always shrouded the sudden emergence, seemingly ex nihilo, of portolan charts. The oldest known example emerged in Pisa around 1290, without any obvious antecedents. This Carta Pisana kickstarted a tradition of amazingly accurate sea charts almost up to modern standards, although as with most other portolans, that accuracy was mainly limited to the Mediterranean and the Black Sea.
A typical portolan chart showed coastal contours and the location of harbours and ports, ignoring virtually all inland features. It would be criss-crossed by straight lines, connecting opposite shores by any of the 32 directions of the mariner's compass, thus facilitating navigation.
After popping up in Italy, portolans became coveted possessions in the seafaring nations of Spain and Portugal, where they ranked as state secrets.
Little or nothing is known of their origins and production, so the working hypothesis among cartographic historians was that portolans were somehow gathered together from the knowledge of medieval European sailors, possibly enhanced with older knowledge from Byzantine or Arab sources.
That hypothesis has now been disproven by Roelof Nicolai, a Dutch geodetic scientist who on 3 March obtained his doctorate degree from Utrecht University for a dissertation titled A Critical Review of the Hypothesis of a Medieval Origin for Portolan Charts.
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