The history of navigation and hydrography, as of other crafts or ‘mechanick arts’, has to be constructed from very imperfect materials. Instruments and charts, as the tools of practical men, were commonly discarded when they were worn out or superseded. The high mortality among original charts makes generalization in the comparative history of cartography hazardous. That no Portuguese charts before 1500 are now known does not allow us to conclude that none was made but only that none has survived. Many of the early charts that have been preserved are decorative examples drawn for royal or other patrons; those by which pilots conned their ships had a smaller expectation of life and are correspondingly rare.
Mark Rosen. Review of "The History of Cartography, Volume 3: Cartography in the European Renaissance (2 vols.)" by David Woodward.