The typical portolan chart was drawn on the finely finished skin of a sheep or, in some cases, a cow.
William Dobbyns, the seventeenth-century `Escheator of the Province of Ulster', visited the premises of a London chartmaker, Thomas Comberford, in 1655 to purchase a portolan chart, and found the cartographer living in abject poverty.
There is no generally accepted comprehensive definition of a portolan chart.
Nonetheless, in this period the portolan chart remained very much centred on the Mediterranean and destinations easily reached from there.
A particularly significant portolan chart of this period of Portuguese exploration of the West African coast is found in an anonymous atlas now preserved in the British Museum (Egerton Collection, ms 73).
On an oceanic voyage, however, especially at higher latitudes, the converging meridians on the physical globe conflicted with the parallel meridians shown on the traditional portolan chart.
The dual latitude scale is visible in his sole surviving portolan chart, which was produced in 1550.
In its purest form, the portolan chart was a highly functional tool for navigation whether in the contained arena of the Mediterranean or later in the open oceans of the world.
Even though portolan charts continued to be produced until at least the end of the seventeenth century, from the middle of that century the portolan chart came to be replaced by printed charts.