Nor are the Chinese characters properly medieval, that for the western God postdating the arrival of Jesuit missionaries. Zheng He's 15th-century travels in the Indian Ocean were indeed sensational, but they were well authenticated. Why diminish them by faking a circumnavigation? Besides, since the map is a copy, there is no way of verifying any original.
I blame the Internet. It is an open house for boosting the egos of crackpot historians, though it does at least facilitate their demolition. Map hoaxers usually fall down not on their inaccuracy but on their accuracy. It was the correctness of the 1440 Vinland map's outline of Greenland that for many years queried its status as the first map to show North America (or at least Baffin Island). Only after much scholarship in the 1960s was its authenticity put beyond doubt. As the Vinland scholar RA Skelton asserted at the time, it remains "the only known cartographic delineation of American lands before the discoveries of Columbus and Cabot."
The Vinland map's historical origin lay in the first known settlement of modern America by outsiders, by the Viking Leif Eriksson in the early 11th century. It is described in the sagas, and archaeological remains survive at L'Anse aux Meadows in Newfoundland.
Since Columbus is thought to have sailed with Bristolians (people from the port city of Bristol in western England) to Iceland and cited "the lands the Bristol captains know" in support of his own expedition, Bristol's claim to the first "discovery" of America since the Vikings remains strong.
Yet so what? As theoreticians of geography point out, it all depends what you mean by discovery. America was "found" by Siberian migrants across the Bering Strait at various times between the 30th and the 10th millennium BC. As for later contacts, a minor industry surrounds the likelihood of Phoenicians, Portuguese and other sailors being blown across the Atlantic before Columbus and never coming back. African and Brazilian rafts may have drifted back and forth carrying seeds, spores and genes with them. That Chinese or other Oriental sailors may have travelled down the coast of Canada is possible. The trouble is, they never told us so.
Discovery requires more than a verifiable fact. It requires understanding what has been discovered, being able to place this mountain, island or coast in a framework of knowledge. That is why skeptics were right to question Columbus as the "discoverer" of America, since to his dying day he was convinced he had reached part of Asia. The real discoverer was Amerigo Vespucci, whose later voyages revealed the new continent for what it was, and from whom it took its name.
Hence Daniel Boorstin's masterpiece of modern geography, The Discoverers, presented the Portuguese Prince Henry the Navigator as a true explorer, merely sitting in his Sagres castle at Cape St Vincent, debriefing his captains and filling the blanks on his maps.
When Europeans heard tell of the New World, they were disappointed to be told by Columbus that "in these islands I have so far found no human monstrosities, as many expected." It was the disappointment of imagination corrected by facts.
Such discoverers were map makers not just of place, but of science, society, economics, the human body and the human mind. In the person of Einstein, Boorstin finally found geography married to physics, "when time and space came together in a single tantalizing riddle." All scientists were geographers at heart. That is why maps are the most sacred tools of science. That is why those who fabricate and abuse them are a menace to the cause of knowledge.