Made by the Marseille school of chart makers, this is the western half of a portolan chart of the Mediterranean. It was likely made as a presentation copy for a noble or merchant of Marseille as it is much more highly decorated than a mariner's portolan chart. The island of Malta bears the Order's cross; if we had the eastern half, we would probably find Rhodes changed in a similar manner.
Mason, Roger, and Pamela Willis. Maps of Malta in the Museum and Library of the Order of St John: A Short Catalogue. St. John Supplies Department, 1989. Page 20 Catalogue number 51