Portolan charts are medieval nautical maps of the Mediterranean whose coastlines are startlingly accurate. The name, applied in the 1890s, comes from the Italian portolano, a book of sailing directions, but surviving charts from before 1500 are mostly drawn on vellum and covered in rhumb lines radiating from compass roses in 16 and later 32 directions. Sailors held one bearing along a line to their destination. Because the lines follow magnetic north, researchers can date undated charts from shifts in magnetic declination. Place names run perpendicular to the coast so a navigator could rotate the chart, and symbols marked hazards: crosses for submerged rocks, black dots for visible rocks, and red dots for shoals.

The oldest surviving chart, the Carta Pisana of around 1280, is already as accurate as those made 200 years later. One theory says medieval sailors compiled dead reckoning data, though compounding errors make that hard to credit. Another, based on cartometric analysis, argues the charts are tracings of older maps assembled as a mosaic of regional charts drawn on something like the Mercator projection, which Gerardus Mercator did not publish until 1569. Many surviving charts were status pieces owned by aristocrats and merchants, while working charts rotted at sea.

• Every chart before 1600 was traced by hand, and the first printed woodcut chart appeared in 1472.
• Copper plate printing, developed around 1477, allowed far finer lines and lettering.
• Mallorcan Jewish cartographer Abraham Cresques made the 1375 Catalan Atlas for the King of Aragon, drawing on Marco Polo.
• Arabic portolan charts from the 15th century and al-Umari’s 14th century encyclopedia show Arab cartographers’ role.
• Portuguese mapmakers added the equator and latitude lines as their ships lost sight of the North Star.