Maps built from reports
A medieval map was usually put together at a desk from information gathered by other people. The chart maker rarely surveyed a coast himself. He collected what pilots, merchants and pilgrims said, compared it with earlier maps and written descriptions, and drew a new version. The process was cumulative, and the quality of the result depended heavily on how good the sources were. Where many sailors passed regularly, reports could be checked against each other. Where few Europeans had been, a map relied on rumor and inherited tradition.
The portolan chart
The most striking product of this process is the portolan chart. These were sea charts of the Mediterranean and Black Sea, and later of the Atlantic coasts of Europe and Africa. The oldest surviving examples are generally dated to around the end of the thirteenth century, and one well-known early chart associated with Pisa is often cited. Because the earliest preserved charts already look quite developed, historians think the type had existed for some time before, though the evidence is thin.
Typical features include:
- a network of lines radiating from points arranged in a circle, usually drawn on the chart in several colors, which helped the user find the direction between ports
- coastal names written inland, perpendicular to the shore, so the sea remained clear
- coastlines drawn in detail, with harbors and hazards marked by symbols
- a scale bar, and flags or small pictures of cities and rulers decorating the interior
- no latitude and longitude lines in the earliest examples
The name comes from the Italian portolano, a written book of sailing directions that listed harbors, distances and hazards along a coast. The chart and the sailing directions were companions: one gave the picture, the other the words.
How they were probably compiled
Exactly how they were compiled remains uncertain. The most common explanation is that chart makers combined sailing directions, compass bearings and estimated distances between ports, recording them as lines on a base sheet, and then adjusted and joined sections to produce a continuous coastline. The magnetic compass was in use in the Mediterranean by the thirteenth century, and it allowed sailors to describe directions between ports in a consistent way. Navigators used dead reckoning, estimating position from speed, heading and time, rather than measuring latitude and longitude.
Charts were drawn on vellum, often in a single skin, and later versions were frequently copied from earlier ones with small updates. The main centers of production were Italian ports such as Genoa and Venice, and the island of Majorca, where chart makers of Jewish and Christian background are recorded. Catalan workshops grew prominent, and the best-known product was the Catalan Atlas of about 1375, a large multi-panel work that joins chart-style coasts with information on Asia and Africa.
Charts as tools and as treasures
Were they used at sea? The answer appears to be yes, at least sometimes, but the evidence is mixed. Navigators would have found coastal outlines and compass lines helpful. At the same time, many surviving charts are lavishly decorated and in excellent condition, which suggests that they were valued objects owned by merchants, nobles or rulers, rather than worn working copies that were often exposed to salt and damp. Simpler working charts probably existed but were less likely to survive.
Using travelers’ reports
For regions beyond the Mediterranean, mapmakers turned to accounts. Itineraries from pilgrims described distances between stops on the way to the Holy Land. Merchants’ notes recorded trade goods and markets. Friars who traveled to the Mongols wrote reports, and the account associated with Marco Polo spread information about Asian lands and the sea routes of the Indian Ocean. Genoese merchants and Venetian sailors could carry fresh information back to their home ports, so the charts of these cities were often among the first to show changes.
A chart maker often had to reconcile contradictory sources. Names were spelled differently, distances varied and some places were reported in more than one location. The maker might record both versions, choose one, or leave the area blank. The result was uneven: the Mediterranean coast, thoroughly known to its sailors, is surprisingly accurate, while the shape of northern Europe, Africa’s coasts beyond known harbors and the interiors of continents are distorted or imagined.
Exploration and the changing map
Maps both reflected and encouraged exploration. When Atlantic voyages along Africa and into the ocean expanded late in the medieval period, new coastlines were added to the charts, and names from the voyages were written along them. The charts gave patrons a visual summary of what was known, and the gaps invited further effort. The earlier world-picture of the mappa mundi, which organized the world by sacred history, was gradually accompanied by maps built on measured experience, though both traditions continued.
Why it matters
Portolan charts show that medieval people were able to build reliable practical knowledge through accumulated experience. They are also a reminder that a map reflects a purpose. A chart made for navigating a coast and a world map made to teach sacred history are both valid answers to different questions.