portulan

portulan, or portolan, chart. This derived its name from port books. It was not only the first sea chart but the first true map of any kind for, based on measurement, it was intended for use rather than display or illustration. Unlike contemporary land maps where size might, for instance, be related to importance, it carried a scale. The earliest surviving example is the Carta Pisana housed in the Bibliothèque Nationale in Paris. Probably of Genoese origin, it is dated about 1275 and drawn on an outstretched sheepskin, the neck (untypically) to the right. The chart gives a remarkably accurate representation of the Mediterranean coastline, derived presumably from sailing directions and the notes and sketches of pilots. There is no graticule of latitude and longitude but a scale of miles is displayed at two places at right angles to each other, presumably to take into account shrinkage. With no projection, the chart is based on bearing and distance, but within the latitude limits of the Mediterranean, convergence of the meridians would be of little account. The bearings are of course magnetic not true, but that again would have mattered little where both compass and sailing directions were also magnetic. The wind-roses display sixteen ‘rhumbs’ corresponding to the cardinal and inter-cardinal points of the compass which are then divided and subdivided by ‘winds’, as directions were then called, and half-winds to provide the network of rhumb lines. By finding the rhumb most closely parallel to the bearing of his destination, and tracing it back to its parent rose, the navigator could thus identify the rhumb, or course, to sail on.

The mathematically precise construction diagrams by which the network of rhumbs was laid down on the chart appear for the first time on the Pisan Chart and continued to be used well into the 18th century. It is worth remembering that this was the period when, largely through the influence of Leonardo of Pisa, the Hindu (‘Arabic’) system of numerals was adopted in place of the clumsy Roman figures, which put arithmetic within the grasp of the ordinary man.

Bibliography

Campbell, T. , ‘Portolan Charts from the Late Thirteenth Century to 1500’, in J. B. Harley and D. Woodward (eds.), The History of Cartography (1987).

Mike Richey