Portolan charts, realistic sea charts of the Mediterranean Sea and the Black Sea that suddenly appeared at the end of the thirteenth century, cannot possibly have been created in Medieval Europe. This is demonstrated by research with which Roel Nicolai (Utrecht University) hopes to obtain his doctorate degree on Monday 3 March. Up till now, it has been assumed that these charts were created by medieval cartographers based on measurements of sailors. But the identity of the creators of these remarkably accurate charts remains a mystery to this day.
The oldest portolan chart in the world seemed to appear out of nowhere at the end of the thirteenth century, and there are no signs of the existence of any simpler, earlier versions. The fact that these charts are so incredibly accurate despite the limited knowledge and measurement instruments has long been seen as a fortunate coincidence. In his research, Nicolai proves that even with the most optimistic assumptions this is, in fact, impossible. "The remarkable thing about his research is that he has studied portolan charts using insights and methods from modern-day geodesy. The results are utterly surprising", says his PhD supervisor, Professor Jan Hogendijk of Utrecht University.
So far, it has been assumed that sailors carefully recorded data about their courses and distances they travelled on busy shipping routes. It was believed that their measurements were compiled in so-called portolans, books with sailing instructions, and eventually processed by cartographers. Nicolai, however, has shown that it is unlikely that the nautical compass was available in time and that navigational methods used at that time were sophisticated enough to determine distances at such a degree of accuracy.
Furthermore, using averages in calculations to increase the accuracy of measurements did not become common practice until the eighteenth century. A mathematical analysis of the oldest surviving portolan has revealed that its source data must have been derived from a portolan chart - instead of the other way around.
Nicolai has also established that portolan charts were drawn on the Mercator projection, or a similar type of projection. A projection is a mathematical conversion of the geometry of the curvature of the Earth to a flat surface. There are no signs so far that indicate that the knowledge required for chart projections was already available in the Middle Ages. It is generally assumed that Mercator projection was not introduced until 1569 by its originator.
Nicolai concludes that part of history needs to be rewritten. "This needs to happen even if I am wrong, because that would mean that they were much further advanced in terms of knowledge in the Middle Ages than we think." This is not likely though, as is demonstrated by the fact that charts of other parts of Europe long remained less accurate than the portolan charts.
An Arabic-Islamic origin is highly unlikely, according to Nicolai. "Although they were scientifically ahead of Europe and had considerable knowledge of chart projections, it has never been convincingly proven that they had the knowledge required for reducing observations made on the Earth's curvature to the flat map surface."
It therefore seems plausible that portolan charts originated from a tradition that is now lost. It is an intriguing question from what culture that tradition stemmed. Further research will be required to establish whether or not Greco-Roman antiquity is a realistic option, says Nicolai.
For other questions: Monica van der Garde, Press Officer of the Faculty of Science, +31 6 13 66 14 38, firstname.lastname@example.org.