The Library of Congress recently played host to "Re-Examining the Portolan Chart: History, Navigation and Science.", an academic conference dedicated to the history and origins of medieval coastal maps.
Starting in 1275, coastal maps (also known as portolan charts) arise that are much more accurate than their predessors.
Despite the prevalence of these charts, it is not clear from either the existing maps or sailors' notes how medieval cartographers aquired such accurate data. The use of Portalan charts began to decline with the era of trans-Atlantic exploration, when cartographers could not accurately account for the curvature of the Earth. [photo]