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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 the advent of the Age of Discovery, they were considered State secrets in Portugal and Spain. They were 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."
Contents and themes 
These charts, actually rough maps, were based on accounts of medieval Europeans who sailed the Mediterranean and Black Sea coasts, and later were used to map coastal resources in the Atlantic and Indian oceans. At the beginning of the Age of Discovery they would describe the coast of Africa, Brazil, India and even past the Strait of Malacca into Japan, knowledge vital for the slow rise to prominence of the English Armada and of Dutch merchants, following in the wake of the Iberian powers. Frequently drawn on sheepskin, portolan charts show coastal features and ports. In earlier days, what could be used as a harbour encompassed more of the coastline than now: as ships were smaller, they might need to seek refuge in a harbour more often, and some ships were intentionally beached for maintenance and repairs. Thus, nearly any protected bay or flat beach might be of interest to mariners, not only for safe harbour but also as previous coastal reconnaissance.
The straight lines criss-crossing many portolan charts represent the thirty-two directions (or headings) of the mariner's compass from a given point. This is similar to the compass rose displayed on later maps and charts. Naming or demonstrating all thirty-two points is called "boxing the compass".
The portolan combined the exact notations of the text of the periplus or pilot book with the decorative illustrations of the conceptual T and O map. In addition, the charts offered a realistic depiction of the shore, and they were meant for practical use by a mariner of the period.
Portolans failed to take into account the curvature of the earth; as a result, they were not helpful as navigational tools for crossing the open ocean. Instead they derived their use in close quarters identification of landmarks. Portolani were useful for navigation in smaller bodies of water, such as the Mediterranean, Black, or Red Seas.
See also 
- Francesco Maria Levanto, Prima Parte dello Specchio del Mare, Genova, G.Marino e B.Celle, 1664. Disponibile anche in ristampa anastatica dell'edizione originale, Galatina, Congedo, 2002. (Italian)
- Konrad Kretschmer, Die italianischen Portolane des Mittelalters, Ein Beitrage zur Geschichte der Kartographie und Nautik, Berlin, Veroffentlichungen des Institut fur Meereskunde und des Geographischen Instituts an der Universitat Berlin, vol. 13, 1909. (German)
- Bacchisio Raimondo Motzo, Il Compasso da Navigare, Opera italiana della metà del secolo XIII, Cagliari, Annali della facoltà di lettere e filosofia dell'Università di Cagliari, VIII, 1947. (Italian)
- Patrick Gautier Dalché, Carte marine et portulan au XIIe siècle. Le Liber de existencia rivieriarum et forma maris nostri Mediterranei, Pise, circa 1200, Roma, École Française de Rome, 1995. (French)
|Wikimedia Commons has media related to: Portolan charts|
- Portolan Charts mini-site, University of Minnesota
- Portolan Charts Samples of portolan charts illustrating the harbors and trade routes of the Mediterranean, 14th–16th centuries, from the Beinecke Rare Book and Manuscript Library at Yale University
- J. Rey Pastor & E. Garcia Camarero La cartografía mallorquina (Spanish)
- Portolan Charts butronmaker