Early Modern Nautical Charts and Maps: Working Through Different Cartographic Paradigms

in Journal of Early Modern History
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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.

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    Point of fantasy (left) and set point (right). The point of fantasy PF is the intersection between the line representing the course, C, and the arc of circle representing the distance d, sailed from the point of departure, PD. The set point SP (right) is the intersection between the line representing the course and the parallel of latitude φ (the horizontal line).
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    The influence of magnetic declination on the point of fantasy (left) and the set point (right). PF1 and SP1 are, respectively, the point of fantasy and the set point, as affected by magnetic declination δ. Notice how magnetic declination only affects the longitudinal position of the set point, thus conserving the observed latitude φ. C is the course; Cm is the magnetic course.
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    The Cantino planisphere (1502) depicts the world as it became known after the exploration voyages in the end of the fifteenth century and beginning of the sixteenth to Africa, Greenland, Newfoundland, Brazil and India. It is usually considered the first “latitude chart,” that is, the first nautical chart representing places according to latitudes. Biblioteca Estense Universitaria (C.G.A.2), Modena.
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    Coastlines of Africa and Brazil in the Cantino planisphere (left), compared with the corresponding outlines in a modern map (right). Notice the eastward displacement of Africa and Brazil, and the longitudinal stretching of Africa. The projection of the modern map is the equirectangular projection centered at the Equator, commonly known as square chart or plate carrée.
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    The effect on the set point of an eastward magnetic declination for a course in the northeast (left) and southeast quadrants (right). The graphs illustrate how the ship’s position SP was marked on an ordinary chart using the compass (magnetic) course Cm, sailed from the point of departure PD, and the observed latitude φ. N is the charts North, from where all courses are reckoned. If magnetic declination were corrected for, the resulting set point SP* would lie to the east (left) or to the west of SP (right), depending on the quadrant of the course. Notice (right) how the eastward displacement of SP (here representing the Cape of Good Hope), relative to its true position SP*, is explained by the eastward magnetic declination in the South Atlantic, as correctly perceived by João de Castro. C is the course, Cm is the magnetic course.
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    Excerpt of a nautical planisphere by Lopo Homem (1554). The black outline represents the coastline of a modern plate carrée map. Although the longitudinal stretching of Africa was reduced, when compared with the Cantino planisphere, the eastward displacement was not completely eliminated. This may be explained by the error, of about 10 degrees, made in the determination of the longitude of Diu. Istituto e Museo di Storia della Scienza, Florence, IMSS, n. 0946.
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    Excerpt of a chart by Pedro Reinel (ca. 1519) with two different latitude scales. The tilted scale near Newfoundland, indicating the direction of geographical north, is only applicable to that region. Bayerish Staatsbibliothek, Munique (Cod Ican 132).
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    Excerpt of the only surviving chart by Diego Gutiérrez (1550), with three different latitude scales: one for Newfoundland, which appears rotated clockwise by an angle matching magnetic declination in the region (center), one for the eastern Atlantic (right) and one for the western Atlantic and Caribbean Sea (left).
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    Grid of meridians and parallels implicit in the Mercator world map of 1569 (left) compared with its own geographical graticule (right). Notice the convergence of meridians in the northern hemisphere and the irregularities of the supposedly straight and parallel lines. Reproduced from Gaspar, “Revisiting the Mercator World Map of 1569” (2016), 8.
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    Excerpt of Mercator’s world map of 1569, depicting part of the Atlantic and Indian Oceans, compared with the coastlines of a modern Mercator’s map (white lines). Notice the longitudinal displacement and stretching of Africa and Brazil, identical to the one in the traditional cartography of the sixteenth century, and the artificial enlargement of the Mediterranean, shared by other planispheres of the sixteenth century. (Berlin, Gesellschaft für Erdkunde, 1891).
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