Geodetic and methodological aspects of the research into the origin of portolan charts

One of most challenging research elements of medieval Mediterranean porto-lan charts is the matter of their elusive origin. This does not concern the origin of the surviving physical charts, but the origin of their geometrical information content, consisting primarily of the coastal outlines. Particularly challenging are the charts’ evident accuracy and their good agreement with a Mercator(-like) map projection. Key questions are how these charts were constructed and on the basis of what measurement data. While questions regarding the medieval usage of the charts, the economic aspects of chart construction and their fabrication process clearly fall in the domain of the historian, those concerning the construc-tion technique, accuracy and correspondence with a map projection have a very considerable geodetic component. It is my contention that geodetic aspects have not been given sufficient attention in the research into the origin of portolan charts. The almost complete absence of his-torical evidence regarding the charts’ origin requires the researcher to tread carefully and pay adequate attention to the methodological aspects of his or her research. I will discuss several examples where the eagerness to come forward with viable explanations has enticed researchers to step off the narrow path prescribed by the scientific method. Geodetic analysis enables the testing of hypotheses related to the geodetic aspects of portolan charts (construction, accuracy and map projection) with more scientific rigour than would be achievable without such tech-niques. The key questions regarding the origin of the charts can, in my opinion, only be answered with some hope of success by a fruitful synthesis of history, geodesy and cartography. Research-ers from the humanities are faced with a particular challenge to get an adequate understanding of geodetic, mathematical and statistical analysis techniques, in order to be able to understand the results such techniques may yield. Part of this challenge is to understand the importance of the selection of the right method or tool. I will illustrate these issues with the choices I made in my research, which resulted in the surprising and, for the (map-) historical community, unorthodox conclusion that portolan charts are not primitive medieval charts, but instead sophisticated, geodetically-constructed cartographic products.