Elements of a Portolan
Vellum
Characteristics in Common
The three portolan charts discussed here are the Albino de Canepa portolan chart from Venice, 1489; the Petrus Roselli chart done in Majorca,1466; and the 1424 Nautical Chart, made by Zuane Pizzigano, a Venetian serving the Portuguese.
The three portolan charts have several characteristics in common. First, they are handwritten and hand painted on vellum.
Along the eastern edge of the three portolan charts are holes which could be used to attach the chart to a wooden roller, around which the chart was to be wrapped, then tied with a thong for carrying, probably in a protective container, on board ship.
Directions
Characteristics in Common
In the fifteenth-century portolan charts considered here the winds had merged with the directions of the compass. The eight principal compass bearings, named for the wind directions, became (in Italian): Tramontana (N), Griego (NE), Levante (E), Syrroccho (SE), Mezzodi (S), Garbino (SW), Ponente (W), and Maistro (NW). (Brown 1949, 126). The figure that shows these directions and winds is sometimes called a wind rose, which is not a good name, as was pointed out by Tony Campbell. The preferable term is compass rose. (Campbell 1987, 395).
The 1489 and 1466 portolan charts both have compass roses; the 1424 does not. It should be noted, however, that the rhumb network of the nautical chart itself is a "compass rose."
Rhumb Lines
Characteristics in Common
Portolans share another characteristic: a network of lines, like the web of a spider, that forms a grid for the map. This grid can easily be seen by looking at the portolan chart from the blank side, against the light, since the vellum is fairly transparent. The hole at the center of the circle which defines the grid is also visible from the back of the portolan chart. Directions are indicated on the portolan charts by this network of lines joined on the points dividing the circle that is a basic element of the portolan chart. The circle is divided into sixteen equal parts, each joined to others through the center point on the opposite side. The lines are called "rhumb lines" or simply "rhumbs."
The rhumbs for the eight primary winds (or directions) are drawn in black (or sometimes gold) ink; the eight half-winds are in green; the sixteen quarter winds in red. Patterns of squares, triangles and parallelograms are visible within the circle on portolan charts.
Winds
Characteristics in Common
The directions indicated by the network originated with the winds, whose names have a long history. In the time of the poet Homer the four winds were the cardinal directions with the north wind, Boreas blowing up rough waves; Notus, the south wind, causing sudden storms; Zephryus, the west and Eurus, the east winds. The prophet Jeremiah also speaks of "the four winds from the four corners of heaven" (Jeremiah 49: 36). The Norse had eight winds: North, Land-North (NE), East-Dawn, Land-South (SE), South-Midday; Out-South (SW); West-Sunset; Out-North (NW).
In the fifteenth century, in early printed editions of Ptolemy's Geographia, twelve "windheads" were commonly shown with faces depicting characteristics of the winds. For example, an old man in the north depicted Boreas.