The map projection of portolan charts

The sudden appearance of portolan charts, realistic nautical charts of the Mediterranean and Black Sea, in the last quarter of the thirteenth century, is considered to be one of the most significant events in the history of cartography. Using analysis techniques available in geodesy, Roel Nicolai showed in dissertation that these charts are mosaics of regional charts that are considerably more accurate than had been assumed earlier. The good agreement of these regional charts with the Mercator map projection is even more remarkable. Map projections were unknown in the Middle Ages and the Mercator projection was developed some three centuries after the appearance of the oldest extant portolan chart. Therefore, virtually unanimous agreement exists among historians of cartography that its map projection must be coincidental. In this article, however, Nicolai shows, using probability calculus, that it is very unlikely that the map projection emerged as an unintentional by-product of the charts’ construction.