Abstract: Portolan charts are realistic and detailed nautical charts of the Mediterranean and Black Seas which appeared suddenly in Italy towards the end of the 13th century. Until now most historians have assumed that these charts are based on measurements of course and distance between ports made by mariners during trading voyages and drawn by a cartographer as if the world were flat. Apart from their evident accuracy, one of the most remarkable characteristics of these charts is their close resemblance to a modern map on the Mercator projection.This study analyses five charts using quantitative geodetic and statistical techniques; this distinguishes it from existing studies, which approach the subject mostly from a qualitative historical perspective.The results are surprising. The charts are demonstrated to be more accurate than has been assumed until now and are shown to be mosaics of five to ten sub-charts, some of which have considerable overlaps. More importantly, the geodetic analysis techniques show that the map projection can only be an intentionally applied design feature of the original charts and not an accidental by-product of the presumed cartographic drawing method, as has been believed until now. The study also shows that portolan charts cannot originate in contemporary Arabic-Islamic civilisation. The consensus view that has existed until now about the origin of portolan charts is thus demonstrated to be incorrect. Their origin must therefore lay considerably further back in time.