Certain arguments against the hypothesis that portolan charts were genuine late-medieval cartographic products

Abstract

ABSTRACT A sample of 12 representations of the Adriatic Sea on portolan charts, created between the late thirteenth century and the late 16th, were subjected to a series of tests in order to examine their navigational applicability. However, the by-products of their results strongly suggest that it is impossible that portolan charts were, in terms of their geometry, genuine late-medieval cartographic products. It was demonstrated that, during the 300 years of their continuous production, their accuracy was not improved, and their anticlockwise tilt was not chronologically aligned with the dynamics of magnetic declination across the research area. Moreover, it was explicitly demonstrated that bearing measurements which could have been performed aboard ship (from sailor's perspective) cannot be used to create charts which display such levels of geometric accuracy, not only in the late medieval period, but in general.