Portolan Charts, Dead Reckoning, and Piloting Techniques

Behind the great voyages of the Age of Exploration was a working set of piloting techniques far less precise than the heroic literature of discovery suggests. A pilot of the sixteenth century did not navigate the way a modern navigator navigates. He could determine his latitude with reasonable accuracy by observing the sun or a star, but he had no practical way to determine his longitude, his compass was off by several degrees, his chip log was an approximate speedometer, and his position estimates, after a long ocean passage, were routinely off by tens or even hundreds of miles. The argument of this page: the actual practice of navigation in the Age of Exploration was a craft of cumulative estimation and constant checking, not a science of exact measurement, and the pilots who made the great voyages were skilled improvisers working within severe technical limits. David Waters’s The Art of Navigation in England and the Netherlands in the Sixteenth and Seventeenth Centuries (1958) and J.H. Parry’s The Age of Reconnaissance (1963) are the indispensable treatments.

The conventional story of the era emphasizes the great navigators — Columbus, da Gama, Magellan, Drake — and treats navigation as a heroic individual achievement. The reality was more institutional and more collaborative. The pilot of a 16th-century ship was a member of a learned trade, trained in a long apprenticeship, supported by printed rutters and tables, working with a master who commanded the ship and a mate who kept the traverse board. The great explorers of the era, including Henry the Navigator, were in many cases organizers and patrons rather than pilots, and they relied on these anonymous professionals to make their voyages possible. The relationship between pilots, masters, and the great explorers is treated in the broader context of the ships and navigation pillar.

Portolan Charts: The Working Charts of Pilots

The portolan chart was the most important practical tool of European navigators in the late Middle Ages and the Renaissance. The earliest surviving examples date from the late thirteenth century, and the tradition flourished in Italy (especially Genoa, Venice, and Pisa), Catalonia, Portugal, and France through the sixteenth century. The name “portolan” derives from the Italian portolano, meaning a collection of sailing directions.

A typical portolan chart depicted a region of coast — usually the Mediterranean, the Atlantic coast of Europe, or both — with a high degree of accuracy in the relative positions of ports, capes, bays, and islands. The chart was crisscrossed by a network of rhumb lines, or loxodromes, radiating from sixteen or thirty-two compass points placed at the center of the chart or at the edges. The rhumb lines allowed a navigator to lay a course from any point on the chart to any other point by simply drawing a straight line and reading off the compass bearing. Combined with the mariner’s compass, this was a powerful and practical system.

Portolan charts of the Mediterranean are remarkably accurate in their depiction of coastlines, often to within a few nautical miles of modern positions. The accuracy was based on the practical experience of pilots, who had sailed the Mediterranean for centuries, and on the cumulative corrections of many generations. The interior of the continents was either left blank or filled with decorative and symbolic information, but the coasts were drawn with great care. Portolan charts were a European specialty, and they were the working charts of pilots from the thirteenth to the seventeenth century.

By the late sixteenth century, portolan charts were gradually being supplanted by Mercator charts, which were based on the cylindrical projection published in 1569 and which became the standard for marine navigation in the seventeenth and eighteenth centuries. The transition was complete by the early seventeenth century in northern Europe, although portolan-style charts continued in use in the Mediterranean for some decades longer. The tradition ended because the new world maps, with their global scope and uniform grid, served the needs of transoceanic navigation better.

Dead Reckoning

Dead reckoning was the bread-and-butter technique of the deep-sea pilot, and it remained central to ocean navigation from the fifteenth to the twentieth century. The method is simple in principle: starting from a known position, the navigator estimates his ship’s position at any moment by adding up the courses steered and the distances run, and then adjusting for current, leeway, and the estimated effects of wind and sea. In practice, the method required much skill and discipline, and much guesswork, and it was the source of many famous errors in the history of exploration. The technique is treated in detail on our dead reckoning long-tail page.

To run a dead reckoning, the pilot needed to know two things: the course steered, and the distance run. Course was read from the compass, ideally adjusted for magnetic variation. Distance was estimated by measuring the ship’s speed and the time elapsed. The standard method of measuring speed was the chip log, developed in the sixteenth century: a wooden board attached to a line with regularly spaced knots, thrown overboard and timed over a fixed interval using a half-minute glass. The number of knots that ran out gave the speed in nautical miles per hour — the origin of the term “knot.” The same line could be used to estimate the distance run over any interval, by counting the knots and converting with the half-hour glass.

To keep track of the course and distance over a watch (typically four hours), the pilot or his mate used a traverse board. The traverse board was a flat board with sixteen or thirty-two holes radiating from a central point, each labeled with a compass point, and a set of pegs. At the end of each half-hour glass, the mate would record the course by placing a peg in the corresponding hole, and would record the distance by placing a peg in a series of holes along the bottom of the board representing the speed in knots. At the end of the watch, the cumulative course and distance could be read off the board and entered in the ship’s logbook.

Dead reckoning was inherently error-prone, and the errors accumulated over time. Magnetic variation was not always known precisely, and even the best compasses could be off by several degrees. The chip log gave only an approximate speed, especially in heavy weather. Leeway — the sideways drift of the ship under the pressure of the wind on the hull and rigging — was hard to estimate accurately. Currents were often unknown, and the navigator had to guess at them. As a result, a ship’s position estimated by dead reckoning could easily be off by tens or even hundreds of miles after a long ocean passage. Pilots therefore checked their dead reckoning by every available means: sightings of land, soundings of the seafloor, observations of birds and sea life, and above all measurements of latitude.

Latitude Sailing

Once a navigator could measure latitude reliably, he could practice “latitude sailing”: sailing north or south to the latitude of his destination, then sailing east or west along that parallel until landfall. This technique was particularly useful in the Atlantic, where the trade winds blow steadily from east to west in the tropics and where the latitudes of many destinations were known approximately from earlier voyages. Columbus used latitude sailing on his return from the Caribbean, sailing northeast to the latitude of the Azores before turning east for Europe. The technique had the great advantage of simplicity: once the navigator had reached the right latitude, he did not need to know his longitude with any precision. It had the great disadvantage of being unable to determine where, along the parallel, the ship was, and it was therefore useless for finding small islands in the open ocean.

To practice latitude sailing, the navigator needed accurate tables of the declination of the sun — the angle of the sun above the celestial equator at each day of the year. The most important such tables of the fifteenth century were the Alphonsine Tables, originally compiled in the thirteenth century under the patronage of King Alfonso X of Castile, and the Tabulae Directionum of Regiomontanus, published in the 1470s. These tables allowed the navigator to convert an observation of the sun’s altitude into a latitude. The tables were not perfectly accurate, and navigators using them were often several degrees off, but they were a vast improvement on guesswork. The Alphonsine Tables and the Tabulae Directionum are treated in greater detail in Owen Gingerich’s The Book Nobody Read (2004), a fascinating intellectual history of the Copernicus–Alphonso connection.

Coastal Piloting

For the parts of a voyage close to land, pilots relied on a different set of techniques: coastal piloting. The pilot would use a portolan chart of the coast he was sailing along, watch the landmarks, sound the depth with a lead line, and follow the rhumb lines from point to point. The lead line was a length of rope with a lead weight at one end; the weight was hollowed out and filled with tallow so that a sample of the seafloor would stick to it. From the depth and the nature of the bottom — sand, mud, rock, shell — the pilot could identify his position even in poor visibility. Lead lines were marked at standard depths (typically at 2, 3, 5, 7, 10, 13, 15, 20, and 25 fathoms) and the leadsman would call out the depth as he sang out. Coastal piloting was a craft, learned by long apprenticeship, and it was the foundation of practical seamanship throughout the Age of Exploration.

Pilots, Masters, and the Division of Labor

The successful execution of a voyage required the close cooperation of two distinct roles: the master and the pilot. The master, often a merchant or a sea captain, was responsible for the overall command of the ship, the management of the crew, and the handling of the vessel. The pilot, often a specialist in navigation, was responsible for the course of the ship, the consultation of charts and sailing directions, and the determination of position. The two were sometimes the same person, especially on smaller vessels, and they were sometimes in conflict. The pilot was typically paid a share of the profits of the voyage, in addition to a fixed wage, to ensure that he had an incentive to bring the ship safely to its destination. The relationship between pilots, masters, and the great explorers of the era is illustrated in the careers of Christopher Columbus, who was both master and pilot on his voyages, and Ferdinand Magellan, who relied on a series of pilots to guide his fleet through the strait at the southern tip of South America.

The Rutter and the Pilot Book

In addition to charts, pilots relied on rutters and pilot books: written collections of sailing directions, descriptions of coasts and harbors, and advice on local conditions. The earliest printed rutters appeared in the late fifteenth century, and the tradition flourished in the sixteenth and seventeenth centuries. The English polymath John Dee, the Portuguese navigator Duarte Pacheco Pereira (whose Esmeraldo de situ orbis, c. 1505–1508, preserved detailed sailing directions for the African and Asian coasts), and the Dutch cartographer Lucas Janszoon Waghenaer were among the most important authors of pilot books. Waghenaer’s Spieghel der Zeevaerdt (1584–1585), translated into English as the Mariner’s Mirror, was one of the most influential pilot books of the era. Rutters and pilot books circulated widely, and they formed an essential part of the practical knowledge of the working pilot.

The Limitations of Piloting Techniques

Despite their sophistication, the piloting techniques of the Age of Exploration were limited. Longitude could not be determined with any accuracy until the eighteenth century, and until then navigators were always uncertain of their east-west position. Charts of the Pacific, the South Atlantic, and the Indian Ocean were full of errors. The instruments themselves were delicate, easily damaged by salt spray, and often inaccurate in rough weather. And the cumulative errors of dead reckoning meant that a captain’s estimate of his position could be off by hundreds of miles. These limitations shaped the great voyages as much as they constrained them. Captains sailed in fleets, made frequent landfalls, and accepted a wide margin of error in their estimates. The pragmatism and improvisation of the pilots who actually used these techniques is one of the great overlooked stories of the Age of Exploration.

The Legacy of the Piloting Techniques

The piloting techniques of the Age of Exploration — portolan charts, dead reckoning, latitude sailing, and coastal piloting — formed the practical core of European navigation for three centuries. They were supplemented, in the eighteenth century, by the marine chronometer and the reflecting sextant, making possible accurate longitude determination. The portolan chart evolved into the modern nautical chart; the traverse board gave way to the patent log; and the chip log was eventually replaced by mechanical and electronic speed logs. But the basic techniques — keeping a log, plotting a course, sighting the sun, sounding the bottom — were the techniques of the Renaissance pilot, and they remained the techniques of the working navigator well into the twentieth century.