A Portuguese portolan nautical chart detailing the coasts of...
Pedro Reinel, c. 1504
Portolan charts are navigational maps based on realistic descriptions of harbours and coasts. They were first made in the 14th century in Italy, Portugal and Spain. With the advent of the Age of Discovery, they were considered State secrets in Portugal and Spain, very valuable in the description of Atlantic and Indian coastlines for newcomer English and Dutch raiding, and later trading, ships. The word portolan comes from the Italian adjective portolano, meaning “related to ports or harbours.” -wikipedia