A critical Review of the Hypothesis of a Medieval Origin of Portolan Charts. Roel Nicolai , 2014

( libro en inglés)

Portolan charts are nautical charts, which appear suddenly in the thirteenth century Mediterranean world, without any discernable development path or predecessors. These charts are remarkable for the high degree of realism and accuracy with which they render the coastlines of their core coverage area: the Mediterranean and Black Seas and the European and North African Atlantic coasts. The shape of the coastlines agrees surprisingly well with a modern map on the Mercator map projection. Until now, scholarly consensus considered portolan charts to be original products of medieval cartographers, based on estimates of course direction and distance measured by seamen during trading voyages. Modern scholars have assumed that the charts were drawn as if the earth were flat. They have attributed the high accuracy of the charts to averaging of multiple measurements on the same routes prior to the mapping process.