Portolan

Portolan, in cartography, designates a portolan chart—a nautical map used by mariners from the late medieval period onward. Portolans emphasize coastal detail, harbor locations, and sailing directions arranged around a network of rhumb lines that radiate from compass roses. They are typically drawn on vellum or parchment and valued for practical coastal piloting rather than for geographic scale or interior landmasses.

Origins and development: Portolan charts originated in the western Mediterranean in the late 13th century, with Genoese, Venetian, and Catalan workshops among early producers. By the 14th to 16th centuries the portolan tradition spread across Europe, including Iberia and the Atlantic-facing nations. A related form, the portolano, referred to a book or collection of charts.

Design and content: The defining feature is the dense lattice of rhumb lines, each representing a constant compass direction and used to plot courses between ports. Compass roses or wind roses mark the directions from which these lines emanate. Coastlines are drawn with careful attention to coastal geometry and include ports, reefs, shoals, and landmarks. Interiors are usually less detailed, reflecting the charts’ practical coastal focus. Latitudinal and longitudinal accuracy varied, and many charts were updated or copied over time.

Production and usage: Portolans were produced by professional cartographers and navigators who relied on firsthand coastal observations and travel experience. They supported dead reckoning and celestial navigation, helping sailors sail along coastlines and between ports. Over time, improvements in projection methods and broader geographic knowledge led to new types of charts, eventually reducing the dominance of portolans in open-sea navigation.

Legacy: Portolan charts remain valuable historical sources for medieval and early modern navigation, maritime trade, and coastal geography. Notable examples are held in major libraries and archives around the world.