Portolan charts

 The word portolan is derived from an Italian term meaning “related to a port or harbor”.  The portolan chart provided a realistic description of harbors and coastlines within the covered area.  They were particularly prevalent in Italy, Spain, and Portugal from the 1300’s through the 1500’s.  During the Age of Discovery, portolan charts of India, the Spice Islands, and the New World were considered state secrets.  Each portolan chart bore a compass rose, along with a series of compass lines showing the direction from one port on the chart to various other ports.  The charts were well suited to navigation of the Mediterranean Sea or for coastwise sailings, but because they failed to account for the curvature of the earth, they were less useful for long distance transits of oceans.  Within their limitations, though, portolan charts provided generally accurate navigational information.  They were drawn, sometimes with great artistry, on vellum (i.e., processed sheepskin).  They were generally kept as scrolls: attached on their right side to a rod so that they could be rolled up and kept in a protected case.  Because they designed for use by mariners at sea, they generally experienced heavy use and, when outdated, were considered of no practical value.  Thus, the vast majority were discarded and few have been left for preservation in museums.  These were the charts, though, that Prince Henry the Navigator continuously updated so as to allow Portuguese mariners like Vasco da Gama to round Africa and reach India in 1497, a journey which at its time was more important to Europe than Christopher Columbus’s voyage to the New World.