| 11 May 2026
Classical Antiquity Origin of Spatial Data Underlying Portolan Charts: How Pietro Vesconte Might Have Assembled the Carta Riccardiana Utilising Maps in Marinus of Tyre’s projection
Abstract. The remarkably accurate representation of coastlines on late mediaeval portolan charts has garnered significant scholarly interest in uncovering the origin of their spatial data, with the prevailing hypothesis suggesting that they are authentic products of their time, created by synthesising numerous shipborne bearing and distance measurements. The pursuit of those answers also prompted the establishment and advancement of a cartometric approach, through which they can be attained with quantitative accuracy. This study is based on cartometric analyses and historical evidence, providing a comprehensive examination of the anonymous Carta Riccardiana portolan chart, presumably created by Pietro Vesconte between 1300 and 1325. It posits a paradigm shift regarding the late mediaeval origin hypothesis, tangibly indicating that the conventional geometry of portolan charts was likely developed by late mediaeval copyist-cartographers who utilised regional maps from classical antiquity, originally made in the projection of Marinus of Tyre calibrated to their mid-latitudes as inputs, which they partially scaled and gradually tilted, using a map in Ptolemy’s first projection as a reference model. Concurrently, the article provides a historical overview of the most significant cartometric studies in the field, critically analysing their unique systemic parameters that have influenced various hypotheses regarding the origin of portolan charts—specifically, the systemically induced similarity with the modern map in the Mercator projection, which is, in the case of the Carta Riccardiana, at least, a misleading notion.
This article examines the previously little-known “Carta Riccardiana” in terms of its accuracy. Combining cartometric analyses with established historical data, this study offers the first clear and comprehensible explanation of how late medieval Italian copyists and cartographers may have compiled this composite data. This research provides a well-founded and detailed refutation of the hypothesis concerning medieval data sources for the creation of portolan charts, thus breaking new ground in scholarship. Maps from the late Middle Ages remain a relatively under-researched topic in the history of cartography to this day. The text is clearly organised, with well-structured chapters. The methodological approach has been explained. There are a sufficient number of illustrations, which are of good quality. The topic effectively bridges the history of cartography with modern cartography, and the article should definitely be published. The article fits very well in the journal, and the title accurately reflects the content of the text. The author possesses the necessary knowledge of history, geography and cartography to write on this topic. The peer reviewer notes only the following minor detail: The “Carta Riccardiana” is attributed to Pietro Vesconte in the article. But who was this man? A biography would certainly be of interest to readers. The methodological approach of modern cartography appears solid. The historical approach and information are fine, except for the missing biography. The English also seems to be fine. However, the peer reviewer is not a native speaker and therefore cannot provide a final assessment.