[A portolan chart of the Atlantic Ocean and adjacent continents].

The word portolan comes from the Italian adjective portolano, meaning "related to ports or harbors", or "a collection of sailing directions". Portolan charts are 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 where they considered to be state secrets. The English and Dutch found the description of Atlantic and Indian coastlines extremely valuable for their raiding, and later trading, ships. The oldest survived portolan is the Carta Pisana, dating from approximately 1296 and the oldest preserved Majorcan Portolan chart is the one made by Angelino Dulcert who produced a portolan in 1339.

Ancient Maps from the Library of Congress. 13th -18th Century Maps.

In the 17th century, maps took a huge leap forward. Mathematical and astronomical knowledge necessary to make accurate measurements had evolved. English mathematicians had perfected triangulation: navigation and surveying by right-angled triangles. Triangulation allowed navigators to set accurate courses and produced accurate land surveys. Seamen learned to correct their compasses for declination and had determined the existence of annual compass variation. Latitude determination was greatly improved with the John Davis quadrant. The measurement of distance sailed at sea was improved by another English invention, the common log. Longitudinal distance between Europe and Québec was determined by solar and lunar eclipses by the Jesuit Bressani in the 1640s and by Jean Deshayes in 1686. With accurate surveys in Europe, the grid of the modern map began to take shape.