This week I have nearly finished the first draft of the chapter called “The Enigma of the Portolan Chart”. I am sure that these charts had a shipboard use; there is enough textual evidence. Sure they were inaccurate – their typical scale 1 cm: 63 km was too small for port identification, that is a counter-argument – but so were nautical guides. Besides, nobody claims that nautical maps were the only device. Marines used other means for dead reckoning, to say nothing of their experience based on intimate knowledge of currents and winds, the flight of birds and the color of waves. In any case, sailors could find a desired haven in the Mediterranean from time immemorial. New devices enabled them to start sailing in the Atlantic throughout the 14th century.
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