What a portolan chart is
A portolan chart is a hand-drawn sea chart, typically on a sheet of parchment made from a single animal skin, showing the coasts of the Mediterranean and Black Sea, and often the Atlantic shores of Europe and Africa. The name comes from the Italian word “portolano”, which meant a book of written sailing directions. In modern usage it refers to the charts that grew up beside those books.
The most striking feature is the network of lines. From several points on the sheet, each marked with a compass rose or a hub of lines, straight lines radiate in sixteen or thirty-two directions, and these overlap to form a web. Each line represents a compass bearing. A pilot could lay a ruler or a pair of dividers from his starting port to his destination, find the line closest to parallel and read off the bearing to steer.
The earliest charts
The oldest surviving example is generally taken to be the Carte Pisane, which is dated to the late 13th century, around the 1290s, and was apparently made in Genoa or Pisa. The chart is not signed. The earliest chart with a maker’s name and date is attributed to Petrus Vesconte of Genoa, who signed charts in the early 14th century, and there is a good number of surviving works from Genoese, Venetian, Majorcan and Catalan workshops during the following centuries.
Historians debate how the tradition arose, since so little survives from before 1300. Some think that charts existed earlier and have been lost, and others point to the written periploi of the ancient world and to medieval pilot books. A famous historical reference is to the voyage of King Louis IX of France in 1270, during which he is reported to have been shown a chart. Such accounts are not firm evidence about the earliest charts, so it is best to say that the beginning of portolan charts lies somewhere in the 13th century or slightly before.
Reading the chart
A portolan chart concentrates on the coast. Place names are written inland, at right angles to the shore, so that the coastline is clearly visible and the names can be read with the sheet turned. Major ports are often in red ink and lesser places in black. Dangerous rocks and shoals are marked with small crosses or dots, and river mouths are drawn, but the interior is mostly empty or filled with decoration, such as flags, cities and rulers, in the more elaborate examples.
A scale bar, divided into units of miles, allowed the pilot to estimate distances. Many charts at first had no latitude scale, since they were built from compass bearings and estimated distances. Later charts sometimes added a latitude scale once navigators began to use the mariner astrolabe and the quadrant.
Accuracy and its limits
The shape of the Mediterranean on these charts is surprisingly good for a drawing made without a mathematical grid. The Black Sea ports, the Adriatic trade routes and the Aegean are all recognisable. Scholars still discuss how the charts were made. Likely sources include compass bearings and dead-reckoning distances recorded by many pilots and compiled by chart makers, and it is probable that a master chart was copied repeatedly by workshops with small modifications. Most surviving charts show strong similarities in outline, which suggests copying more than fresh measurement.
Portolans do have limits. Because the sheet is treated as a flat plane, they work best over limited areas, and errors would grow with distance. They also show nothing about depth, current or tide beyond a few warnings. They were aids to a pilot who already knew the sea, not a substitute for his knowledge.
Who used them
The users were pilots and captains in the busy trade of the medieval Mediterranean seafaring world, including merchants from Genoa, Venice, Pisa, Barcelona and Majorca, carrying goods such as grain, cloth, spices and timber. The Venetian Arsenal and other shipyards supplied the vessels, and the charts guided them between ports.
Some charts were also made as luxury objects. Rulers and merchants commissioned decorated charts that were displayed rather than carried on deck. The famous Catalan Atlas of about 1375, attributed to the Majorcan workshop of Abraham Cresques, is a world map and an atlas that uses portolan features, and it is a presentation work, not a working pilot’s tool. Working charts were handled in all weather and wore out, which is one reason so few early ones survive.
Change and legacy
In the 15th century, Portuguese and Spanish voyages down the African coast and across the Atlantic extended the area that charts needed to cover. Chart makers added the coast of West Africa and, after 1492, parts of the Americas. Charts with a latitude scale appeared, and the cartographers gradually moved towards the use of a graticule of latitude and longitude lines.
The portolan tradition faded as the Mercator projection and printed sea atlases took over. The older charts remain important as historical documents. Their coastlines, names and conventions show how medieval pilots saw the Mediterranean, and they stand as the first widely used family of navigational charts in Europe.