<P> 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 . </P> <P> As Leo Bagrow states: "...the word (" Rhumbline ") is wrongly applied to the sea - charts of this 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' ." </P> <P> 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 Age 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 harbours 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 harbour but also for coastal navigation . </P> <P> The straight lines shown criss - crossing many portolan charts represent the thirty - two directions (or headings) of the 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 eras, 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 ." </P>

Who produced the oldest signed and dated portolan still in existence