Portolan chart of the Central Mediterranean. Portolan charts are navigational maps based on compass directions and estimated distances observed by the pilots at sea. They were first made in the 13th century in Italy, and later in Spain and Portugal, with later 15th and 16th century charts noted for their cartographic accuracy. With the advent of widespread competition among seagoing nations during the Age of Discovery, Portugal and Spain considered such maps to be state secrets. The English and Dutch, relative newcomers, found the description of Atlantic and Indian coastlines extremely valuable for their raiding, and later trading, ships. Attributed to Joan Oliva active 1580-1615. Joan Oliva was the most prolific member of a large family of Catalan chart makers. Charts signed by at least sixteen members of the Oliva family are recorded, with dates between 1538 and 1673.