Portolan Charts : Encyclopedia of Geography

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Portolan Chartsportolan charts

Portolan charts were produced in Mediterranean lands primarily during the 13th through 16th centuries. Originally single-sheet maps that were drawn on rolled scrolls of sheepskin and used by sailors in the Mediterranean region, they were soon joined by portolan atlases that typically covered the Atlantic coast of Europe and the Black Sea as well as the Mediterranean and that likely served a more decorative function. In both formats, however, portolan charts are striking for their relatively “modern” appearance and their apparently accurate representation of coastlines, leading the early 20th-century cartographic historian Charles Beazley (1904) to call portolan charts “the first true maps” and to assert that “in them, true cartography, the map-making of the civilised world, begins” (p. 161).

Perhaps the most distinctive feature of the portolan chart is the network of rhumb lines—usually colored red, green, and/or brown—that cross over ocean space and in some cases continue across land space. Together with a portolan—a list of directions for sailing from one port to another along a coastline—these rhumb lines were used for navigation. For instance, a sailor wishing to cross the Mediterranean could find his source and destination cities on the portolan chart, note the rhumb line whose angle corresponded to the angle of a line drawn between the two cities, determine the corresponding compass heading, and then sail by dead reckoning along that heading until he reached the opposite coast. On reaching the coast, assuming that the boat did not land precisely on target, the sailor could use an accompanying portolan to navigate coastwise to reach his ultimate destination.

Portolan charts also are notable for their simplistic projection, in which longitude lines are drawn as straight, parallel lines without any compensatory adjustment in the spacing of latitude lines (e.g., as is achieved in a Mercator projection). This projection, and the accompanying system of navigating by dead reckoning along a compass heading as one traversed a conceptually placeless ocean, was suitable for trans-Mediterranean travel, where north-south distances are relatively small, currents and tides are minor, magnetic variation is insignificant, and storms are rare during the sailing season. However, attempts to transfer this projection and its corresponding technique of navigation to longer-distance ocean voyages would be disastrous, and thus, it seems likely that the frequent presence of rhumb lines on 17th-century world maps was purely decorative or, perhaps, was intended to reference an earlier era of Mediterranean voyaging.

Despite the break between the navigational practices enabled by and reflected in the portolan chart and those in later maps, some scholars have noted that portolan charts nonetheless embody a perspective that prefigures modern conceptions of territory and mobility. As on a modern map, the ocean on a portolan chart is typically devoid of emotive, natural, or human signifiers (e.g., sea monsters, ships, toponyms). Instead, the ocean is represented as an empty space that can be rationally crossed by sailors following geometric logic and the natural law of magnetic north. In this sense, the portolan chart established the norm of the ocean as an easily crossable but empty (or non-exhaustible) and hence nonpossessable space, which was later to be codified by 17th-century jurists such as Hugo Grotius and which has gone on to inform the modern law of the sea. Likewise, the coastline on portolan charts is represented as a series of destinations (names written along the coastline) whose social significance is buttressed with numerous flags. Here, too, a modern norm is prefigured, as civilization is associated with naming and political territorialization. Behind the coastline, the interior—especially on Iberian portolan charts—is depicted as a fecund space of opportunity, filled with bustling human settlements and exotic nature, prefiguring a colonial mindset where the interior of distant lands—the space to be encountered after an ocean voyage—is presented as inaccessible yet ripe with opportunity (as well as dangers). Finally, islands on portolan charts are represented as territorially unified entities wherein the seemingly incontrovertible boundary between land and sea is used to naturalize a political boundary between the internal space of the state and the outside space beyond its control. This depiction foreshadowed the ideal of the territorial state that was to become a political (and cartographic) norm on land in the centuries that followed.

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Map of the Western Mediterranean from a portolan atlas by Battista Agnese, ca. 1544 Source: Library of Congress, Geography and Map Division. Available at http://memory.loc.gov/ammem/ gmdhtml/gnrlagn.html.

In short, while Beazley might well be accused of ethnocentrism in his identification of certain maps as being “true” and in associating them with a singular “civilization,” his broader point—that the spatial conceptions inscribed in the portolan chart foreshadowed perspectives on space that were to inform emergent systems of exploration, colonialism, and merchant capitalism—is well taken. Long celebrated for their beauty and “accuracy,” portolan charts should also be appreciated for the ways in which they inscribed ideas about divisions between fixity and movement, land and ocean, and territory and an external space that is immune to possession. These idealized divisions, expressed so succinctly in the portolan chart, went on to guide the spatiality of the modern era.

Philip E. Steinberg