They suddenly appeared in the thirteenth century:portolan charts, stunningly accurate nautical charts with coastlines almost as detailed as those on modern charts. They were made using techniques that were not known at all at the time, says Roel Nicolai in his dissertation. The medieval people must have copied the maps from mysterious older sources, probably without understanding how accurate they really were.
It is a popular mystery that often comes up in historical novels. The sudden emergence of so-called 'portolan cards' in late thirteenth century Europe. These maps depict coastlines that are astonishingly similar to those shown on modern nautical charts. What makes them so mysterious is that they suddenly appeared in Italy and later in Spain, apparently without clear predecessors.
Portolaan cards were used in shipping from at least the fourteenth century. Presumably the beautiful maps also did well as a decorative piece on the walls of wealthy merchants on the Mediterranean. The oldest known portolan map appeared around 1290 in the Italian city of Pisa.
Mosaic
But the maps are so accurate that they couldn't have been made in the Middle Ages at all, at least not in this way. This is what Roel Nicolai, head of geodesy in daily life (the science concerned with determining the shape and dimensions of the earth) at Shell, concludes in his thesis.
Nicolai shows that the maps are so accurate because a form of map projection has been applied; a mathematical translation of the round earth on a flat surface. But that technique was not known at all in the Middle Ages (at least as far as we know now). The first useful method was not introduced until 1569 by the Flemish mapmaker Gerardus Mercator, almost three hundred years after the first portolan map appeared. “The investigated portolan maps seem to have been made with the Mercator projection. But I can't give 100 percent certainty about that," says Nicolai.
“They were definitely drawn on parchment in the Middle Ages, there is no doubt about that,” he continues. “But probably the artists did that without really understanding how accurate the maps actually were. We immediately recognize the shape of the Mediterranean on a map, but in the late Middle Ages that image was not yet so firmly anchored. Nobody knew the exact shape of the sea and how the coastlines ran.”
Moreover, Nicolai was able to demonstrate that the maps were not made in one piece, but are in fact a kind of mosaic. “You can see clear differences in scaling and orientation between different geographic areas on portolan maps,” he explains. “That's clear evidence that the cards were made up of separate cards. And that also indicates that the medieval draughtsmen were not familiar with the technology behind it.”
Shoved down
The common thought among medievalists is that portolan maps were based on portolani :written sailing instructions that included ports, distance measurements, directions and sailing directions. Information from different sea voyages was combined, averages were calculated and the medieval mapmakers were able to make accurate maps based on this. According to these researchers, the great similarities with modern maps are mainly due to chance.
Nicolai shrugs off all those assumptions. He tried to imitate the medieval trick, but the accuracy of a map produced from medieval navigation measurements turned out to be at least a factor of ten worse than that of the portolan maps. In addition, the Middle Ages had no knowledge of arithmetic means to improve accuracy. Those methods were only discovered and applied towards the end of the seventeenth century.
“The strangest thing is that no older maps have been found anywhere that could have served as an example for medieval copyists,” says Nicolai. Suddenly they were there. There is no clear development. And it wasn't until the nineteenth century that mapmakers were able to match the accuracy of portolan maps again.
“It's very special that the medieval sailors, probably without understanding the technique or fully realizing how accurate the maps were, still saw their potential,” says Nicolai. "The cards were actually used for shipping and proved to be extremely practical to use.
“There are good reasons to believe that they were used even before the thirteenth century. The Crusades in the eleventh century saw a revolution in sea transport of armies and horses to the Holy Land. Those horses could come along because the transport suddenly went four times as fast, while the ships remained the same. This may also be due to the discovery of these portolan cards.”
Almighty Civilizations
If the medieval mapmakers were probably only copying these enigmatic maps, the big question is:where did they come from? Nicolai cautiously states that they were probably acquired through trade with Constantinople. “But it's unlikely they were originally made there,” he adds. “As far as is known, the Byzantines did not add very much to the traditional scientific knowledge of antiquity. They were primarily a repository and conduit for ancient Greek and Arabic knowledge. And why would the Byzantines map the French or British coastline, for example? That was far outside their territory.”
Did the knowledge to make them perhaps – like so many medieval scientific innovations – come from the Arab world? Arab astronomers were more advanced than the Europeans in determining geographic longitude and latitude, among other things. But even the accuracy that the Arabs put in their maps could not match that of the portolan maps, according to Nicolai. An Arab origin seems equally unlikely. Based on what we know about the Greek and Roman state of science, an origin in classical antiquity is also not obvious.
“The knowledge of map projection used must be older than the Middle Ages. Perhaps we should check whether our image of the state of science in antiquity is correct, says Nicolai. “But let's not immediately speculate on all-powerful lost civilizations. The origin of portolan cards is something we have to figure out step by step.”