Clues in the Errors
Hessler began to look for clues within the portolan charts themselves. Borrowing the morphometric techniques he used to track movement of the Alpine butterflies’ spots, he transferred each point from a modern Mercator map of the Mediterranean onto the equivalent point on the oldest
portolan chart at the Library of Congress. According to carbon dating of its calfskin substrate, this document was created sometime between 1290 and 1350.
The resulting grid on the portolan chart was slightly distorted in various small ways — not surprising, given the imprecise sailing data with which the mapmaker likely had to work. But it was also fairly consistently rotated by 8.5 degrees counterclockwise. Why?
Hessler suspected the skew was an artifact of compasses, which had arrived in Europe from China not long before the map was created. He knew that compasses respond to the Earth’s magnetic field, which is generated by molten iron moving in the Earth’s outer core. But magnetic north doesn’t line up perfectly with true north, the point where the Earth’s axis hits the surface (and above which the North Star sits). The difference between magnetic and true north, called magnetic declination, varies slightly with time and place, reflecting shifts in the flow of the molten iron. Modern mapmakers correct for declination by adding or subtracting the appropriate number of degrees for particular locations.
Working from compass measurements but not correcting for declination could cause just the kind of rotation Hessler’s analysis revealed. So he went on the hunt for information about historic declination and found a book that provided mathematical models estimating how the declination has changed over time. He consulted the estimates for around 1300, and bingo: 8.5 degrees. Now Hessler had strong evidence that the mapmaker, relying on sailors’ records, didn’t correct his measurements for declination. After all, Mediterranean mariners had no need to worry about how their charts were oriented with respect to the globe — they just needed a reliable guide for the region.
Hessler’s detective work turned up one other clue to the mapmaker’s method: Although the rotation was close to 8.5 degrees throughout the chart, it varied a bit. Italy was rotated only 6 degrees, while the Black Sea was rotated up to 8.8 degrees. That suggested the mapmaker created the chart using different observations made at different times. The result “highlights one of the most interesting problems that historical cartographers faced,” says Hessler. How did the mapmaker decide which records to draw on? “Faced with all this data from different places and times, how did they know what was more accurate?” he adds.
Hessler analyzed other portolan charts in the same way, and each time, the correspondence with his book’s predictions was nearly exact. Between 1300 and 1350, the declination in the Mediterranean fell by 2 degrees — and in keeping with that change, portolan charts drawn by the end of that period were about 2 degrees less rotated. By 1500, declination was back at 8.5 degrees, and so were most of the charts Hessler examined. Over the next 150 years, the declination shifted again, to 11 degrees, and the rotation of the charts followed suit.
To track how portolan charts’ accuracy changed over time, Hessler drew again on the methods he used to quantify butterflies’ evolutionary relationships. As with the butterflies’ wings, he imagined each chart drawn on a metal plate and simulated bending it to move the landmarks on the medieval chart to meet their locations on a modern map. The less energy required to distort the metal into the new shape, the more accurate the chart.
Curiously, he found, in the first several decades after the first portolan chart was drawn, subsequent charts’ accuracy declined a bit. Hessler speculates that the first portolan mapmaker’s technique spread quickly, but those who adopted his methods initially lacked his skill, so their efforts were less precise. As mapmakers’ skills steadily improved over the next two and a half centuries, so did the accuracy of their maps.
Across the Atlantic
As he pieced together answers to some of the questions that had vexed historians, Hessler says, “So many things amazed me: how much mapmakers of early portolan charts knew, how they updated their data so quickly, how accurate their compasses were, how geographic information flowed around the world in ways that we don’t understand.”
Portolan charts paved the way for the age of exploration. Now, sailors could travel down the coast of Africa and around the cape. Eventually, maps were made that extended across the Atlantic to the New World. But paradoxically, the age of exploration that followed the creation of the portolan charts eventually led to their downfall, as increasingly sophisticated techniques of shipbuilding and mapmaking made them obsolete.
The problem was the portolan mapmakers’ lack of a systematic way of reducing the spherical Earth to a flat map. That was of little consequence for short journeys but mattered much more when sailing longer distances. In 1569, the Belgian geographer and cartographer Gerardus Mercator created his method for presenting a spherical world on a flat map, the one familiar to us today. This mapmaking technique, though it stretched and compressed landmarks and distances between them, had the great advantage that a straight compass course was represented by a straight line on the map.
Mercator’s projections began to be used to sail the open seas by the early 1800s, by which time portolan charts had pretty much disappeared. But their importance is undeniable. “The development of these maps revolutionized how people perceived space, much like Google Earth has done in our lifetimes,” Hessler says. “Understanding how the technology was developed gives us insight into how we got here, and perhaps into where we’re going.”