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portolan chart, also called harbour-finding chart, compass chart, or rhumb chart, navigational chart of the European Middle Ages (1300–1500). The earliest dated navigational chart extant was produced at Genoa by Petrus Vesconte in 1311 and is said to mark the beginning of professional cartography. The portolan charts were characterized by rhumb lines, lines that radiate from the centre in the direction of wind or compass points and that were used by pilots to lay courses from one harbour to another. The charts were usually drawn on vellum and embellished with a frame and other decorations. Of the roughly 130 portolans surviving, most were made in Italy or Catalonia and a few in Portugal. The Italian portolans tend to encompass only western Europe and the Mediterranean basin, but some Catalan charts can be considered world maps.
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