It's hard to describe the appearance of a portolan chart - the medieval answer to the modern Admiralty chart - if you haven't already been able to see one. Very few early examples are freely available online but you can find a chart of 1403 here: http://brbl-dl.library.yale.edu/vufind/Record/3521236. Probably originating in the 13th century, though not everybody agrees about that, the portolan charts present the
**Former map librarian of the British Library (1987-2001), since 1993 Tony Campbell has been chairman of Imago Mundi Ltd, in which capacity he acts as co-ordinator for the biennial International Conference on the History of Cartography. He is working on Pelagios 4 as the expert adviser on portolan charts.