How did medieval sailors draw coastlines so well, with a web of compass lines?

Nobody knows exactly how medieval mapmakers drew these sea charts so well: the best guess is web of lines first, coast copied from an older chart second. Portolan charts are the earliest known type of nautical chart, and the oldest examples date from the late 13th and early 14th centuries in the Mediterranean region. They are recognisable by their coastal focus, networks of colour-coded straight lines in 32 directions, scale bars in portolan miles, and place names written perpendicular to the coast.

The strange thing is the quality. Their most perplexing features are the very realistic coastlines and the lack of any visible evolution: the oldest charts are already highly developed, and later ones did not become more accurate. Some historians think the lines come from compass observations, but the tilts of the westernmost and easternmost regions do not match the period's magnetic declination, which implies a magnetic compass did not establish the geometry. Some scholars believe instead that the charts were made from a large body of shipborne bearing and distance data observed through dead reckoning.

The web of lines radiates from wind roses and shows 16 compass directions, which became 32 from around 1450. Drawing probably started with the windrose lines, and then the mapmaker copied the coastal outlines from an earlier chart. Explanations of how to draw one survive only from the 16th century, so medieval techniques can only be inferred.

Were they used at sea? Portolans ignored the curvature of the Earth, so they were not helpful for crossing the open ocean and were replaced by Mercator projection charts. Some scholars even argue that many or most were ornamental objects meant to show off the owner's worldliness; one merchant seems to have commissioned a chart of the Mediterranean he had travelled as a souvenir, not a guide.