The Secret Maps

In the 15th and 16th centuries, Portuguese nautical charts count among the most strategic objects in Europe. They contain crucial information about ocean routes, winds, and newly explored coasts. Some were kept secret like state documents. Spies, copyists, and merchants regularly attempted to obtain clandestine copies for rival kingdoms.

During the age of discovery, Portugal develops a vast maritime network connecting Africa, India, Brazil, and Asia. Nautical charts then become instruments of economic and military power.

Portuguese portolan charts contain extremely valuable data: port positions, marine currents, dangerous zones, and commercial routes. Their dissemination was strictly controlled to prevent rival kingdoms from accessing the same strategic advantages. Despite these protections, several maps are copied clandestinely. The famous Cantino Planisphere is probably obtained through industrial espionage in the early 16th century before being transmitted to the court of Ferrara in Italy.

These documents then reveal geographic information that Portugal sought precisely to keep secret, notably about African coasts and new Atlantic routes. Cartographers sometimes mixed exact data with deliberately imprecise elements to complicate enemy copies.

Map-making also required advanced astronomical knowledge. Navigators combined star observations, approximate latitude calculations, and empirical experience of ocean winds. At that time, possessing an accurate map could directly modify the economic balance between European powers. Nautical charts thus become objects at the frontier between science, commerce, and political intelligence.