World Wide Words

From the Weird Words section

Portolan

Pronounced /ˈpɔːtələn/

Before the days of accurate surveying and printed charts, ships’ captains maintained vellum manuscript books containing hand-drawn charts and details of the routes between ports together with descriptions of harbours and sea coasts. These were called portolans.

Portolan maps usually show a detailed coastline with the names of all the coastal towns, estuaries, inlets, bays, promontories, rivers, underwater obstacles, reefs and other prominent features accompanied by the lines of the compass directions of the prevailing winds. ... The portolans were drawn on parchment and were almost always coloured, since the colours provided vital information for the captains, such as showing town sizes and their importance, and indicating whether the coastline was friendly or not.

Historic maps of Armenia, by Rouben Galichian, 2004.

Though the ancient Greeks had similar charts, known as periploi, the earliest of the portolans that survive, of Italian, Genoese or Catalan origin, date from the fourteenth century. They were for the western Mediterranean, but others were prepared for the trading routes worldwide in the two centuries in which they remained in use before engraving and improved mapmaking made them obsolete.

The name is a version of the original Italian portolano, derived from porto, “port”, and meaning “pilot book”. Portolano sometimes appears as a variant spelling in English. Its written equivalent in Italian was the confusing compasso.