Cartography Word of the Day: Portolan

With a look defined by a network of converging and diverging lines, “portolanos” instantly suggest sea travel in the great age of exploration.

A portolano, or “portolan” chart, was indeed a method for navigation. Used chiefly during what we call the European “Medieval” time, from approximately 1300 CE to as late as 1620 CE, thier design seems to suggest they were used for locating oneself or for plotting thier course across the sea.

Detail of a portolan chart from Battista Agnese’s 1543 Atlas, now in the Library of Congress (illustration excerpt from Robinson, Sale, and Morrison’s Elements of Cartography, 2nd ed., Wiley & Sons, 1978)

The defining characteristc of the portolano are the extensive network of rhumb lines or loxodromes, or lines of constant direction. the arrangement of them, usually diverging from a number of centers, radiating sixteen to a center, suggests directions of the compass. The showed approximately the same area; the Meditteranean and Black seas with thereto unseen accuracy, and the Atlantic Ocean from the Straits of Gibraltar to Ireland with very poor accuracy.

Other notable features of the chart are comparatively extreme detail on the coasts and locations of coastal cities but little or no detail of inland areas (frequently decorated with coats-of-arms of the nations surrounding as well as sometimes picture of royalty), and an orientation of the rhumb network at about 11º, which suggested to Raisz the involvment of the magnetic needle, which must have shown an approximate variation from true North of about that amount.

The methods of construction and exact definition of the portolano are still open to debate, but they seem to indicate the beginning of the transition of cartographic compliation away from the fanciful and guesswork and toward a professional mien.

About 130 portolanos are known to survive today, mostly made by Italy, Portugal, and Spain.

References:

• Britannica Online article on portolans
• General Cartography, 2nd ed, by Erwin Raise, pp 17-19, McGraw-Hill, 1948
• Elements of Cartography, 2nd ed, Robinson, Sale and Morrison, pp 21-24, Wiley & Sons, 1978