Pigafetta remarked in his chronicle about Magellan’s cirumnavigation of the globe: “the Captain General, who knew that he must make his passage through a strait much concealed, as was seen in the treasury of the King of Portugal, in a chart made by that most excellent man Martin de Bohemia”. Below is the original Italian text:
Il Capitano Generale che sapeva de dove fara la sua navigazione per uno streto moldo ascoso, como vite ne la thesoraria del re de Portugal in una carta fata per quello excelentissimo huomo Martin de Boemia, &c.
This mysterious map seen by Magellan was mentioned again by Antonio de Galvao, who was governor of the Portuguese colony in the Eastern Indies in the 1530s. In his later years, he wrote a history on the Portuguese voyages of discovery, and in it, included a very strange passage:
in the year of 1428, they said that the Crown Prince dom Pedro […] was in Roma & Venice, [and] brought from there a Map of the World that had the whole of the Earth & the Strait of Magellan was called Dragon Tail…
The Dragon Tail
Apparently this was the map seen by Magellan.
This Cola do dragam as Galvao called it appears in several world maps drawn between 1440 and 1489 (well before Columbus set sail to discover America in 1492). They all display a peninsula on Asia that named the Dragon’s Tail.
Historians Paul Gallez and Dick Edgar Ibarra Grasso analyzed the maps and built a convincing case suggesting that this peninsula is actually America.
The 1448 map of Benedictine monk Andreas Walsperger bears on the southern tip of the Dragon’s Tail (where Patagonia would be), an inscription that reads “Hic sunt gigantes pugnantes cum draconius”—here live giants who fight against dragons. Gallez interpreted this as an allusion to the Patagons hunting enormous sea elephants.
The 1470 Zetiz world map carries a virtually identical legend (“Homines gigantes pugrant cum draconibus”).
The Nova Cosmographia per totum circulum map reconstructed by the scholar Dana Durand from a manuscript compiled by Brother Fredericus ca. 1450, has the legend “dy Risen vechten und streiten wider dy lint wurm”—Giants fighting and quarreling against the lindworm [*] ; once again a reference to giants and ‘dragons’. The image below shows a detail of this map and the text:
[*] Note on the Lindworm: A wingless dragon of Northern Europe, from Swedish ‘lind’ (flexible body) and ‘orm’ (serpent).
The giants and Patagonia
These coincidences confirm that giants were not something made up by the mid-fifteenth-century cartographers; they were considered as a geographical fact and as such were recorded on their maps.
It is quite likely that Magellan may have seen one of these maps or a copy of them, and known that by sailing down the Dragon’s Tail he would reach the land of giants, by the strait. This foreknowledge would have provided a framework into which the Patagons would neatly fit, and be pronounced as ‘giants’ when encountered by Magellan’s expedition.
The question that will remain unanswered: who drew the original map? Who was the navigator who crossing the Atlantic charted Patagonia's wild coast and put it on a map later redrawn by all these cartographers?
I am not inclined to believe that they were the Chinese as Menzies suggests in his book 1421. Maybe a covert Portuguese expedition in the early 1400s? After all, there is some speculation about Brazil's early discovery (before 1492). 
 Gallez, P., (1981). Walsperger and His Knowledge of the Patagonian Giants, 1448. Imago Mundi 33 (1981): 91-93. And (1980). Los gigantes de Patagonia en la cartografía medieval. Revista de la Universidad Nacional del Centro 10/11, 141-147. Tandil (Argentina).
 Galvao, Antonio. (1731) Tratado dos Descubrimientos Antigos E Modernos. Livraria Civilização Editora, 1987. pp. 77.
 Time Magazine. Archaeology: Before Columbus or the Vikings. Friday, May. 24, 1968
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©