Portolan chart

Portolan or portulan 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 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 they could be loxodromes only if the chart was drawn on a suitable projection.

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