Centre for the History of Science and Technology

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.

The first nautical charts used to navigate in the Atlantic, in the late Middle Ages and
Renaissance, were identical to the portolan charts of the Mediterranean, only differing
in their geographic limits. With the introduction of astronomical navigation, and
when a scale of latitudes was for the first time added to these charts, it quickly became
obvious that their geometry had to be modified in accordance with the new
navigational methods. The resulting hybrid model represented a major breakthrough
in the nautical cartography of the Renaissance, marking the beginning of the evolution
form the “maps based on routes”, to the Ptolemy’s system of geographical coordinates.
Using cartometric analysis, and taking into account the navigational
methods used in the 16th century, it will be shown that these charts were projectionless,
in the sense that they were constructed plotting directly on the plane the observed
latitudes, courses and distances, as if the Earth were flat.

For more than two centuries much has been written about the origin and method of
construction of the Mediterranean portolan charts; still these matters continue to be the object
of some controversy as no one explanation was able to gather unanimous agreement among
researchers. If some theory seems to prevail, that is certainly the one asserting the medieval
origin of the portolan chart, which would have followed the introduction of the marine
compass in the Mediterranean, when the pilots start to plot the magnetic directions and
estimated distances between ports observed at sea. In the research here presented a numerical
model which simulates the construction of the old portolan charts is tested. This model was
developed in the light of the navigational methods available at the time, taking into account
the spatial distribution of the magnetic declination in the Mediterranean, as estimated by a
geomagnetic model based on paleomagnetic data. The results are then compared with two
extant charts using cartometric analysis techniques. It is concluded that this type of
methodology might contribute to a better understanding of the geometry and methods of
construction of the portolan charts. Also, the good agreement between the geometry of the
analysed charts and the model’s results clearly supports the a-priori assumptions on their
method of construction.

A numerical model using the concept of multidimensional scaling, generalized to distances and directions measured on the surface of the Earth, is presented and tested, with the objective of simulating the main geometric features of early nautical charts. Starting with a sample of points defined by their latitudes and longitudes, the process consists in rearranging their positions in a plane so that the differences between the initial (spherical) and final (planar) distances and directions between them are minimized. The geometry of the Cantino planisphere (1502) is simulated and the output is compared with the geographic grid implicit to the original chart, with satisfactory results. The model proved to be an effective and easy-to-use research tool and may be used, not only for simulating and assessing the various factors affecting the geometry of early nautical charts, but also for educational purposes e.g. illustrating the properties of map projections

The results of a cartometric analysis of the early sixteenth-century Portuguese chart of the world known as the Cantino planisphere are presented and discussed. The focus is on errors affecting the geographic lines and the representation of the coastlines. The outputs of three types of analyses are considered: a quantitative assessment of scales (scales of latitude and scales of distance); a qualitative evaluation of the implicit grid of meridians and parallels underlying the representation; and an assessment of the accuracy of latitudes. From these, details were obtained about the chart's construction, some contradicting previous interpretations, which allowed connections to be established between the depiction of certain areas on the chart and contemporary voyages of exploration. It is concluded that the geometry of the Cantino planisphere is fully explained by the use of the cartographic methods of the Renaissance once the effect of magnetic declination is taken into account.