Portolan chart

Portolan or potulan 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. The word portolan comes from the Italian adjective portolano, meaning "related to ports or harbors", or "a collection of sailing directions".

Portolan's rhumblines 

Portolan maps all share the characteristic rhumbline networks, which emanate out from compass roses located at various points on the map. These, better called "windrose lines", are generated by observation and the compass, and designate lines of bearing (though not to be confused with modern rhumb lines and meridians). 

To understand that those lines should be better called "windrose lines" one has to know that portolan maps are characterized by the lack of map projection, for cartometric investigation has revealed that no projection was used in portolans, and those straight lines could be loxodromes only if the chart was drawn on a suitable projection. 

As Leo Bagrow states: ".. the word ("Rumbline") is wrongly applied to the sea-charts of the period, since a loxodrome gives an accurate course only when the chart is drawn on a suitable projection. Cartometric investigation has revealed that no projection was used in the early charts, for which we therefore retain the name 'portolan'."

Contents and themes

These charts, actually rough maps, were based on accounts by medieval Europeans who sailed the Mediterranean and Black Sea coasts. Such charts were later drafted for coastal resources in the Atlantic and Indian oceans. At the beginning of the Ago of Discovery, charts had been made for the coast of Africa, Brazil, India, and 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 vellum, portolan charts show coastal features and ports. During that period, smaller ships could use more of the coastline as harbors than in the present era. They might need to seek refuge more often, and crews intentionally beached some ships for maintenance and repairs. Thus, mariners sought to learn of protected bays or flat beaches, not only for safe harbor but also for coastal navigation. 

The straight lines shown criss-crossing many portolan charts represent the thirty-two directions (or headings) of mariner's compass from a given point, with its principal lines oriented to the magnetic north pole. Thus the grid lines varied slightly for charts produced in different ears, due to the natural changes of the Earth's magnetic declination. These lines are similar to the compass rose displayed on later maps and charts. "All portolan charts have wind roses, though not necessarily complete with the full thirty-two points; the compass rose ... seems to have been a Catalan innovation."

The portolan combined the exact notations of the text of the periplus or pilot book with the decorative illustrations of a medieval T and O map. In addition, the charts provided realistic depictions of shores. They were meant for practical use by mariners 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, being replaced by later Mercator projection charts. Portolans were most useful in close quarters' identification of landmarks. Portolani were also useful for navigation in smaller bodies of water, such as the Mediterranean, Black, or Red Seas. 

History

The oldest extant 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. This led towards two families of Portolan charts: the ones that are purely nautical and those that are nautical geographical (with information and details of the inland). The Catalan portolan charts are of this second type, being usually made in Majorca. 

The origins of portolan charts are obscure, having no known predecessors despite their accuracy compared to other maps of the period. One study concludes that portolans originated from earlier charts drawn on what is now called the Mercator projection. The study confirms the widely accepted idea that portolans are mosaics of smaller charts, each with their own scale and orientation.