Portolan charts are highly realistic medieval charts that show remarkably accurate coastlines of the Mediterranean and the Black Sea. They emerged suddenly, without any predecessors or a clear developmental path, in Italy during the thirteenth century. There is broad scholarly agreement that these charts are original creations of European medieval culture. However, corroborating evidence is lacking, and a convincing explanation of the method of their construction has so far not been provided. In this essay it is demonstrated by means of geodetic analysis that the overall shape of the coastlines corresponds closely to that on a modern map based on the Mercator projection. It is further demonstrated that this correspondence cannot possibly be due to chance. Consequently, the existence of a Mercator or Mercator-like map projection on portolan charts is incompatible with the assumed medieval origin of these charts. Portolan charts are far more sophisticated than has hitherto been recognized. Their construction was well beyond the capabilities of cartographers from either medieval Europe or the Arabic-Islamic world. This conclusion serves to reopen the question of the origins of the geometric data and the construction methods that until now have appeared to underlie medieval portolan charts.

Paper presented at the First International Workshop on the Origin and Evolution of Portolan Charts, Lisbon, June 2016. Abstract: One of most challenging research elements of medieval Mediterranean portolan 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 construction 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 historical 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 techniques. 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. Researchers 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.

In this thesis a methodology for the systematic geometric analysis and modeling of pre-Mercator nautical charts is proposed and tested, aiming at contributing to better understand their geometric properties and methods of construction. The suggested approach involves the application of a series of cartometric techniques: georeferenc-ing, on the basis of a sample of control points of known geographic coordinates; the interpolation of the geographical graticules implicit to the representations; the assessment of scales of distance and latitude; the assessment of the navigational accura-cy, by comparing the latitudes, directions and distances measured on the charts with the corresponding exact values, affected by magnetic declination; and the identifica-tion of the routes underlying the charts’ construction. A numerical model was devel-oped using the concept of ‘multimensional scaling’, here generalized to distances and directions measured on a spherical Earth, to simulate the main geometric features of the charts. To estimate the spatial distribution of the magnetic declination in various times two sources were used: the historical observations made by D. João de Castro in 1538 and 1541, and a recent geomagnetic model. The methodology of cartometric analysis proposed in the thesis proved to be effective and accurate. Five Portuguese charts from circa 1471 to 1504 were analyzed using the full set of techniques mentioned above. The analysis revealed the coexistence of two distinct cartographic models in the nautical cartography of the time: the ‘portolan-chart’ model, based on magnetic directions and estimated distances, used to represent the Mediterranean, the Black Sea and Western Europe; and the ‘latitude chart’ model, based on astronomically-observed latitudes, used to represent Brazil and Africa. His-torically relevant conclusions about the construction details and the sources used in the compilation of the charts were drawn. Some of the most interesting refer to the Cantino planisphere, whose cartographic information was found to be compiled from several sources of distinct origins, times and accuracies. Certain peculiarities of its geometry, especially the location of Greenland, the distorted shape of Africa and the orientation of the Mediterranean, are shown to be the result of the navigational and charting methods of the time, under the influence of magnetic declination. Concerning the standards adopted in the Iberian pre-Mercator nautical cartography, important conclusions were drawn on the type of distance scales and the length of the degree of latitude, some of them contradicting the results of previous studies. One of the most interesting is that the various lengths of the degree adopted in the charts of the time had little navigational impact, being only an echo of the traditional models of the Earth and of the political disputes between Portugal and Spain. The numerical model developed for the purpose of simulating the main geometric features of the charts proved to be a valuable research tool and was used, not only for producing the simulations presented in the thesis, but also for quickly assessing the influence of the various factors affecting the geometry of the charts. The variety and richness of the conclusions drawn in the present research eloquently confirm the utility and effectiveness of the proposed methodology. Hopefully they will arouse the interest of others and help recognizing the need for a multidisciplinary ap-proach in the study of old nautical charts.

This contribution represents a focused insight within a larger research project dealing with the Ottoman nautical atlas of Piri Reis: the Kitab i Bahriye (1521 and 1525-1526). Taking as a point of reference the Ligurian Sea, and more specifically the Tuscan coast, the Ottoman atlas has been compared with the medieval nautical charts and with the early sixteenth-century production (Lepore, Piccardi, Pranzini, 2011). In the quest for sources that may have inspired or influenced the Ottoman Admiral, we have identified and more closely analysed the authors of several nautical charts and atlases and of isolarii. This article focuses on cartometric issues relating to mediaeval nautical cartography (Portolan charts). More specifically, it analyses the 1403 chart of Francesco Beccari conserved in the Beinecke Rare Book and Manuscript Library of Yale University. This chart represents a crucial turning-point in the development of nautical cartography: namely a refinement in the drawing of the coasts relating to the Atlantic, with corrections that were partially retained in subsequent production. It also features a graduated scale of latitude which is considered a later interpolation. The technique behind the construction of the portolan chart (with a rapid incursion into the symbolism connected with the place name colouring) will be analysed in detail, placing it in relation to other selected models that have been conserved. The aim is to contribute to the debate on the techniques of construction of the portolan charts.

This paper explains how, following the introduction of astronomical navigation, the transition between the portolan-type chart of the Mediterranean and the latitude chart of the Atlantic was facilitated by the small values of the magnetic declination occuring in western Europe during the Renaissance. The results of two preliminary cartometric studies of a sample of charts of the sixteenth century are presented, focused on the latitude accuracy in the representation of northern Europe and the evolution of the longitudinal width of the Mediterranean and Africa.

The earliest known historical clue to what might be a primitive nautical chart is in a medieval manuscript of c.1200, the Liber de existencia riveriarum et forma maris nostri mediterranei, which describes the Mediterranean Sea. The body of the manuscript consists of an account of the Mediterranean and Black seas and the Atlantic coasts of Europe and northern Africa organized in 45 regional sections, each introduced by a short text setting out the orientation and size of the region, followed by a portolan-like listing of contiguous coastal localities with the distances between them. These are complemented by additional tracks across the open sea (pelagic tracks) for which both distances and directions are provided. Two distinct types of directions can be distinguished: those not affected by magnetic declination, which indicates that they were probably determined by astronomical methods, and those affected by systematic errors that could only have originated in observations made with a marine compass. It is suggested that some of the pelagic courses in the Liber were compiled from an existing chart or sketch based on astronomical directions, which may have been used as a general reference for the work. The implication is that the genesis and technical evolution of the medieval portolan chart were more complex than has hitherto been thought by map historians, who have based their analyses on the few extant exemplars from the end of the thirteenth and beginning of the fourteenth centuries, all based on compass directions.

Of all the technical and scientific developments that made possible the European maritime expansion, the nautical chart is perhaps the least studied and understood. This fact is very surprising as it was with the information contained in those charts, and later imported to geographical maps and atlases, that the newly discovered lands were first shown to the European nations. There was, however, a deep incompatibility between these two cartographic paradigms—the nautical charts and the geographical maps—which remained unsolved throughout the sixteenth century and beyond, despite the attempts to harmonize the technical principles of Ptolemy’s Geography with the advances of nautical cartography. An eloquent symptom of such incompatibility was the difference between what was understood as an accurate depiction of the Earth, in the eyes of cosmographers and geographers, and what was considered by the pilots as an accurate nautical chart. The misunderstandings around these issues during the early modern period and the unsuccessful attempts at reconciliation were, in great part, the cause for some polemics among cosmographers, cartographers and pilots, such as the conflict in the Casa de Contratación around the charts of Diego Gutiérrez, a fact not entirely understood by historians. At the core of the difficulty lies the circumstance that only in the present day has the true nature of the nautical chart, as a navigational tool, started to be clarified. How the differences between geographical maps and nautical charts contributed to shape the History of Cartography in various periods, and how they are related to conflicting scholarly objectives and practices, is the subject of this essay. We will show, using the results of cartometric analysis, that not only were those artifacts constructed using different principles and with different purposes, but that they belonged to incompatible cartographic paradigms, and we will argue for the relevance of this fact for the history of science.