in Seville – 1521 in Cebu) was the chief pilot-cosmographer (astrologer) of the Armada del Maluco, the fleet commanded by Ferdinand Magellan in 1519.
Confident of his unique skills he applied for the post of Pilot Major of Spain upon the death in 1512 of its holder, Amerigo Vespucci.
Magellan and his Portuguese countryman Faleiro had planned the expedition together, but when the latter developed mental problems the king removed him from the enterprise.
He was suspected of being involved in the failed mutiny attempt in April 1520 in Port St. Julian and was tortured by strappado, but afterwards was allowed to continue his service as cosmographer.
Twice, he was able to calculate accurately, using astronomical measurement, the longitudes of two places, Port San Julian at Patagonia and Homonhon Island in the Philippines.
At San Julian, he brought out his instruments, to quote Tim Joyner, "to test Faleiro's system for using conjunctions of the moon with the planets to determine longitude.
This lengthy possession by de Mafra of San Martín's papers has led to the belief voiced by geographer Donald D. Brand and embraced by Magellan historiographer Martin Torodash, who is cited by Philippine religious historian John Schummacher, S.J., for Brand's dictum, that the eyewitness account of Ginés de Mafra is nothing more than his recall of what he read from San Martín's papers.
This has led to historians overall disregarding the testimony of de Mafra which has overarching importance in Age of Discovery geography of the Philippine region.
In fact, because de Mafra's observations may reflect as well the genius of San Martín, his testimony becomes doubly important.
These two names were first used by Maximilianus Transylvanus in his report of the circumnavigation contained in a letter to Cardinal Matthäus Lang, the archbishop of Salzburg.