This map of the Black Sea takes its coastal outline and names from a portolan chart, but it omits the navigational rhumb lines. It is from a manuscript island book, the Insularum illustratum, by Henricus Martellus Germanus, who worked in Florence ca. 1480-96.
To the historian of late medieval and early modern European cartographyl the portolan charts are fundamental documents, if mysterious in their origin and precocious in their precision. Their importance has long been acknowledged, and "The First True Maps" was the enthusiastic title of an article by Charles Raymond Beazley in 1904. More recently, Armando Cortesao considered the "advent of the portolan chart ... one of the most important turning points in the whole history of cartography." Alberto Magnaghi went further, describing them as a unique achievement not only in the history of navigation but in the history of civilization itself. For Monique de La Ronciere the work of the first named practitioner, Pietro Vesconte, was so exact that the Mediterranean outlines would not be improved until the eighteenth century. In terms of the economic history of cartography, Vesconte and his contemporaries may have been the first, in the plausible opinion of a recent writer, "to pursue mapmaking as a full-time commercial craft.
From the earliest extant copies, probably a little before 1300, the outline they gave for the Mediterranean was amazingly accurate. In addition, their wealth of placenames constitutes a major historical source. Their improvement over the Ptolemaic maps relating to the same area is obvious at a glance, and the North African coast with its clearly defined Syrtes is the most striking advance. Moreover, the Ptolemaic maps began to circulate widely through Europe only in the fifteenth century, by which time the portolan charts were well established. Though a linear scale was implied on Ptolemy's maps by their grid of longitude and latitude, the medieval sea charts were the first cartographic documents to regularly display one. This should be contrasted with the history of European topographical mapping, which shows that the first local map since Roman times to be drawn explicitly to scale was a plan of Vienna dating from about 1422.8 As P. D. A. Harvey further points out, virtually no local maps produced during the period under discussion, that is up to 1500, made "the slightest attempt at consistency of scale.
An even greater gulf divided the portolan charts from the medieval mappaemundi, the cartographic content of which was largely shaped by their theological message. It is worth recalling that the earliest known portolan chart is thought to be almost exactly contemporary with the Hereford world map. It cannot be claimed, of course, that the portolan charts were totally free from what today we call superstition, but neither were medieval sailors. Yet Prester John, the four rivers of paradise, the mythical Atlantic islands, and other legendary features found on some charts are all placed in the little-known interior or around the periphery. The continental coastlines that constitute the charts' primary purpose are in no way affected. The unidentified author of the Genoese world map of 1457, whose depiction of the Mediterranean is based on the portolan charts, neatly sums up the chartmakers' attitude: "This is the true description of the world of the cosmographers, accommodated to the marine [chart], from which frivolous tales have been removed."
The medieval sea chart is the clearest statement of the geographic and cartographic knowledge available in the Mediterranean. Occasionally the coverage was extended to the East, as in the case of the Catalan atlas. Contact with China, however, ceased after the mid-fourteenth century with the collapse of that Tatar empire at which the Polos had marveled. But in the West the portolan charts of the fourteenth and fifteenth centuries provide the best, and at times the only, documentation of the first chapter of Renaissance discovery-the exploration of the Atlantic islands and the charting of Africa's entire west coast. The Spanish and Portuguese seaborne empires whose foundations were to be laid by Christopher Columbus and Vasco da Gama were the fruits of these preparatory voyages.
The medieval mappaemundi are the cosmographies of thinking landsmen. By contrast, the portolan charts preserve the Mediterranean sailors' firsthand experience of their own sea, as well as their expanding knowledge of the Atlantic Ocean. They are strikingly original, signaling, as Gerald R. Crone pointed out, a "complete break with tradition." Whatever their antecedents might have been, these cannot be identified with any confidence today; but this is only one of the many unanswered questions these documents pose. How was the prototype constructed and when? How were copies manufactured for some four hundred years without steadily increasing distortion? Did the Catalans influence the Italians, or vice versa? And most fundamental of all, what was their function?
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