Bearings Straight (II)


Early Sailing Charts

As predecessors go, portolan charts are an impressive lot. In addition to having held the mathematically superior Mercator projection at bay for a century or two after its initial presentation in 1569, they attract a far greater following among map historians, who recognize them as a distinct cartographic genre. And as this chapter observes, portolan charts not only taught mariners to rely on sailing charts but also left a legacy of geographic detail for later mapmakers.

It’s easy to treat portolan charts as both enigma and innovation. They appeared suddenly in the late thirteenth century with crisscrossed rhumb lines and abundant place names, all in sharp contrast to the prevailing religious cartography typified by small, sparse, eastup world maps centered on Jerusalem. Unlike the medieval mappae mundi, which were largely inspirational, portolan charts were practical tools for crossing open waters. And unlike the well-documented publication of Gerard Mercator’s world map, the murky origin of the portolan charts has invited much speculation, not likely to be resolved, about whether Italians or Catalan Spaniards crafted the ultimate prototype, which historians have yet to find.

In their handbook of cartographic innovations, map historians Helen Wallis and Arthur Robinson list four key characteristics of portolan charts. Foremost is the web of intersecting rhumb lines, typically originating on the circumference of a circle, around which sixteen equally spaced points represent the eight principal wind directions (N,NE,E,SE,S,SW,W,and NW) and the eight half-winds (NNE,ENE, ESE,... ) of the mariner’s compass (fig. 2.1). On most charts the circle is readily apparent in the points at which rhumb lines converge like spokes in a wheel. Look closely at the portolan chart in figure 2.2, which covers the western Mediterranean, and you’ll see traces of a large circle centered at the middle of the chart and touching the top and bottom edges. Rhumb lines also converge at the circle’s center, and at the lower right, over North Africa, one of the sixteen intersection points on its perimeter serves as a compass rose. On some oblong portolan charts, like the example in figure 2.3, adjacent circles cover eastern and western parts of the map.



Closer inspection of the chart in figure 2.2 reveals a second distinguishing trait: an abundance of closely spaced, hand-lettered place names perpendicular to the shoreline and always inland, to avoid conflict with coastal details. Additional labels over water identify small islands. Because chartmakers inked in these names one after the other in a continuous coastwise sequence, labels appear inverted where the shoreline reverses direction. A third characteristic is color-coded names and directions. More important places, labeled in red, stand out from less significant neighbors, lettered in black. Color also reduces confusion among rhumb  lines, inked in black or brown for the eight principal winds, in green for the eight half-winds, and in red for the sixteen interspersed quarter-winds. The fourth trait is a functional generalization that rounds minor coastal irregularities, overstates bays and headlands, and uses crosses and dots to point out rocks and shoals. Except for lavishly decorated versions intended for royal collectors, portolan charts showed what mariners needed to know and not much else.

Inked on treated animal skin called vellum, portolan charts withstood rough handling at sea better than paper navigation charts, which did not become common until the eighteenth century. Animal hides were especially suited to the Mediterranean’s pronounced east–west elongation. After splitting the calf’s or sheep’s skin along the stomach, the vellum maker removed the appendages and head but kept the neck, which formed the noticeably narrowed end of a large oblong drawing surface. The typical portolan chart is drawn on a single skin with the tapered end pointing west, to accommodate the Mediterranean’s narrowed reach toward the Atlantic. The flesh side of the skin provided a smooth writing surface; younger animals, with fewer scars, were preferred. Treatment included soaking the hide in lime, scraping off hair and flesh, stretching over a drying frame, rubbing with pumice to smooth the surface, and massaging with chalk to create a neutral, off-white background. Although the charts could be rolled for easy storage—like a thin leather glove, vellum is flexible— some were mounted on wood or cardboard to prevent shrinkage.

Medieval chartmakers are not wholly anonymous. Tony Campbell, the British Library’s former map librarian who wrote the chapter on portolan charts for the multivolume History of Cartography, lists forty-six individuals known to have produced portolan maps or atlases before 1500. Especially noteworthy are Pietro Vesconte, a Genoese mapmaker whose earliest known nautical map is a 1311 chart of the Mediterranean and the Black Sea,and Giovanni da Carignano, a Genoese abbot once credited with the earliest dated portolan chart, believed to have been drafted around 1300. No one questions Carignano’s authorship of the chart, which was destroyed during World War II, but comparison of photographic copies with other maps of the period reveals places names not widely known or used until the 1320s. Cartography was not Carignano’s vocation, but by the late fourteenth century demand for sailing charts was supporting specialist chartmakers in the Italian ports of Genoa and Venice as well as their Catalan counterparts of Barcelona and Majorca.

At least a few medieval chartmakers benefited from an edict endorsing navigation maps. In 1354 King Peter of Aragon ordered all ships to carry two portolan charts, the second perhaps as backup if the other were ruined. Peter’s ordinance reflected the charts 'value as navigation aids as well as the consequences of a ship foundering or getting lost. The earliest surviving record of a chart used at sea is an account of a 1270 voyage by France’s King Louis IX. Because of rough weather the captain decided to seek shelter at Cagliari, in Sardinia, and brought out a chart to reassure the frightened monarch that land was nearby.

The oldest known portolan chart is the Carte Pisane, drafted around 1290 in Genoa but named after Pisa, where it was discovered. Shown schematically in figure 2.3, the chart measures 20 by 41 inches (50 by 104 cm),encompasses the Mediterranean and part of the Black Sea, and includes all four characteristics of its genre. Separate circles anchor two networks of rhumb lines. Hidden on later charts, the circles here are inked in and obvious. Beyond the circles are several squarish grids, with no apparent role. Although seventeenth-century mapmakers used temporary grids, sketched in pencil, as guides for copying features from other charts, erasable pencils were not available until the sixteenth century. Tattered edges and missing fragments of vellum toward the upper right reflect repeated handling. Acquired in 1839 by the Bibliothèque Nationale, the Carte Pisane is a lucky survivor. Campbell, who uncovered fewer than two hundred pre-1500 portolan charts in public and private collections, dedicated his chapter to “the thousands of ordinary charts that served their purpose and then perished.”

Although scholars have yet to uncover a detailed description of medieval chartmaking, they’re certain that portolan charts were copied by hand from existing charts. Microscopic analysis of inked lines and tiny pinholes indicates that chartmakers first laid out the rhumb circle by using dividers (an instrument with two sharp points for transferring exact dimensions) to mark its center and sixteen equally spaced points on its circumference. Using the pinpricks as guides, artisans inked in the network of rhumb lines with pen and straightedge. They then transferred the shorelines from a master map, but exactly how remains a mystery. Some chartmakers apparently forced a fine powder through small holes in a master pattern placed over the fresh vellum, some used a crude form of carbon paper, and some are alleged to have anchored the master map on a transparent frame or table, placed the vellum on top, positioned a strong light source on the opposite side, and traced coastlines and other features directly. Still others might have been exceptionally good at visual transfer—what my cartography students call “eyeballing it.” Once the shorelines were laid down, transferring the place names was a straightforward yet painstaking process.

The prevalence of copying raises questions about the ultimate master chart: who crafted it, when, and how? Although map historians hold little hope of identifying the first chartmaker, they’re certain the prototype portolan chart—if indeed there was only one—was compiled from maps of smaller areas based on books of sailing directions called portolani. Written to help seamen find ports and avoid hazards along the Mediterranean coast, these medieval Italian sailing guides have an equally obscure origin. Although sailors had been taking notes on coastal navigation for over a millennium, pilot books with distances and bearings as well as shoreline narratives emerged at about the same time as the portolan charts. Or perhaps a bit before: extant portolan charts greatly outnumber surviving portolani, which were not decorated and never caught the fancy of royal collectors.

Wary of untested assumptions, Jonathan Lanman, a retired medical researcher and map collector, compiled sailing maps from the Lo Compasso de Navigare, a pilot book from the late thirteenth century, and the Parma-Magliabecchi Portolano, from the fifteenth century. Although fragments of older sailing guides exist, these were the earliest, most complete examples he could locate. To assess the cartographic validity of their sailing directions, he reconstructed the Mediterranean shoreline by chaining together straight-line segments based on distances reported in Italian sea miles,1 sea mile equaling 0.67 nautical miles (1.23 km), and bearings based on a thirty-two-point compass rose. Rotation of the resulting plots and careful alignment with the present-day shoreline revealed a realistic representation of the Mediterranean coast. Despite less than perfect matches, Dr. Lanman demonstrated that the information in the sailing guides was fully adequate for drawing dependable portolan charts.

Curious about the roles of map projection and magnetic declination, Lanman examined the geometric accuracy of the Carte Pisane and a second chart drawn in 1559 by Matteo Prunes, a Majorcan chartmaker. Although cartographic historians generally consider portolan charts “projectionless” for lack of a graticule, Lanman suggested they were “drawn on a square grid” noticeably skewed as a result of magnetic declination. Although evidence of an overt grid is speculative— Lanman’s argument rests largely on small squares within the rhumb circles of the Carte Pisaneand few other charts—locally reliable shapes reflect at least an unconscious appreciation of conformality, a key property of the Mercator projection. Researchers who have confirmed this proto-conformality (my term) include Waldo Tobler,a pioneer in computer cartography, who observed a strong similarity between a 1468 chart by Majorcan chartmaker Petrus Roselli and an oblique Mercator projection. And in a cartometric analysis of twenty-six charts, Scott Loomer, a cartography instructor at West Point, found strong correlations with conformality and straight loxodromes—exactly the properties needed for reliable navigation over open waters. Because a medieval sailing chart typically covered a small area, its informal, ad hoc projection was not a serious weakness.
Some historians recognize the 4 by 4 grids within the Carte Pisane’s rhumb circles as linear scales, running vertically as well as horizontally. Intersecting grid lines divide a distance of roughly 200 miles into four equal parts, and horizontal and vertical scales that are similar—or would be if the interior elements were perfect squares—signify the chartmaker’s unconscious pursuit of conformality. At least that’s how map historians interpret the grid. Unlike the scale bars on contemporary maps, scales on portolan charts didn’t specify distance.

The role of the magnetic compass in the compilation and use of portolan charts remains contentious. Did the compass play an important part in the compilation of early prototypes,or did it merely contribute to a more effective use of sailing charts and later updates? According to Lanman, the orientation of map features accords well with historic trends in magnetic declination. But in Tony Campbell’s view the jury is still out. The magnetic compass was in use by the thirteenth century, but it’s questionable whether instruments available around 1290 were sufficiently reliable to have contributed significantly to either the Carte Pisaneor contemporary portolani. Magnetic variation, which could provide a clue, is difficult to reconstruct, especially before 1600. Although a westward increase in magnetic deviation in the region was apparent until the seventeenth century, local magnetic anomalies thwart a reliable reconstruction of local details. What’s certain is that chartmakers corrected their bearings after better measurements became available around 1600.

Four centuries of portolan charts document European exploration of the African, American, and Asian coasts as well as advances in seamanship in England, Portugal, and what is now the Netherlands. For an appreciation of these improvements, compare the vague rendering of the Mediterranean coast on the Carte Pisane (see fig. 2.3) with the more detailed shorelines in the 1544 map by Venetian mapmaker Battista Agnese (see fig. 2.2). The more recent map is a double-page spread from a portolan atlas in the U.S. Library of Congress’s cartographic collection. Several of the atlas’s nine charts encompass the east and west coasts of North and South America, and a world map depicts the global journey of Ferdinand Magellan’s crew—the explorer died en route— as well as a meandering course from Spain to Panama and then down the coast to Peru. As the charted world expanded beyond the Mediterranean, navigators found the atlas format, with maps on vellum bound in leather, convenient for protecting their charts as well as accommodating new knowledge too detailed for a single map.

Expansion of detailed coverage into the Atlantic encouraged cartographers to correct scale disparities between the charts 'Atlantic and Mediterranean sections. Because pilot books for these areas had been compiled independently, with no attempt to resolve inconsistencies, early portolan charts underestimated distances along the North Atlantic coast by 16 to 30 percent relative to distances in the Mediterranean. These discrepancies persisted until 1403, when Francesco Beccari responded to feedback from mariners with a new chart that also corrected another reported deficiency. As the Genoese chartmaker’s inscription reveals, “It was several times reported to me by many owners, skippers and sailors proficient in the navigational art that the island of Sardinia ... was not placed on the charts in its proper place. Having listened to the aforesaid persons I placed the said island in the present chart in the proper place.” Several decades passed before other chartmakers adopted Beccari’s adjustments.
Since portolan charts were constructed from bearings and distances rather than a determination of geographic coordinates, they lacked indications of latitude and longitude and explicit projections. In the sixteenth century, latitude scales made a halting appearance on sailing charts, but even then, they were simply laid over the framework of rhumb lines, rather than integrated with it. Figure 2.4,a chart from a 1582 atlas by Spanish cartographer Giovanni Martines, shows this disconnect. The north–south and east–west lines on the chart do not represent particular meridians or latitudes; they are simply the extensions of these cardinal directions from the various wind roses. Even so, a navigator with dividers could determine his destination’s latitude, use a quadrant or astrolabe (instruments for measuring latitude at sea) to guide him north or south to the right parallel, and then sail due east or west to the intended port. Mariners call this parallel sailing.



Medieval chartmakers who included a graticule were usually more precise. The result was an equi rectangular projection, characterized by evenly spaced parallels intersecting evenly spaced meridians at right angles. On an equi rectangular rendering of the globe, north– south scale is constant and correct throughout, but because the meridians do not converge, east–west scale is generally distorted, particularly at the poles, where the projection stretches a point into a line as long as the equator.

As figure 2.5 illustrates, an equi rectangular framework maps the sphere onto a cylinder sharing the same axis. Sphere and cylinder touch along one or two standard parallels, which are lines of true east–west scale. In the tangent case, with the sphere merely touching the cylinder, the equator is the map’s sole standard parallel, and distortion is lowest in its vicinity. In the secant case, the cylinder penetrates the sphere along two standard parallels, with equal but opposite latitudes, and distortion is lowest in their vicinity. This effect is illustrated in the right part of figure 2.5,where an equi rectangular projection “secant at 45°”has standard parallels at 45°N and 45°S. The map’s grid cells are compressed noticeably in the east–west direction because a degree of longitude at 45° is only 70.7 percent as long on the globe as a degree of latitude. Both cases, tangent and secant, have true north–south scale throughout, and their meridians are identical in length.


Because distortion increases with distance from a standard line— as I show later, meridians and other lines can also be “standard”—an equi rectangular projection with a standard parallel at 39 N, the approximate latitude of Majorca, provides a generally low-distortion portrait of the Mediterranean, no part of which lies more than nine degrees away from this line of true east–west scale. Although the map distorts distances and bearings, the deformation is less troublesome than the imprecise positions of the features on medieval sailing charts.

Devised around AD 100 by Marinus of Tyre, a predecessor of the Egyptian astronomer-geographer Claudius Ptolemy, the equi rectangular projection is the oldest and most straightforward cylindrical projection. Although Ptolemy deemed it suitable only for regional maps, equi rectangular grids simplified the plotting of points identified by latitude and longitude. In the late fifteenth century Portuguese chartmakers began to use a square-grid variant called the plate carrée or plane chart—essentially the tangent case of the equi rectangular projection. With true scale along the equator and all meridians, the plane chart (fig. 2.6) was less suitable for navigation maps of the Mediterranean than an equi rectangular projection secant within the region, but by 1500 explorers were venturing across and beyond the Atlantic Ocean and their royal and mercantile sponsors needed maps global in scope. As this latter audience grew in size and importance, the plane chart provided a convenient framework for cartographic milestones like Martin Waldseemüller’s 1516 Carta Marina and Diego Ribero’s 1529 Carta Universal. Although ill-suited for plotting rhumb lines and estimating bearings, the plane chart was nonetheless useful for parallel sailing and sketching coastlines. Because tradition-bound mariners learned to live with its distortions, the plane chart dominated nautical charting for over a century after Mercator introduced his demonstrably superior 1569 world map.



To be followed next week




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