[SFR]

Probably an educated guess; with a pretty decent understanding of navigation and math one could understand there was probably a giant space between the west coast and China, that and a little inside knowledge from others, ie by 1542 Cabrillo had sailed to San Diego.

[SFR]

ie by 1542 Cabrillo had sailed to San Diego.
except this map was done in 1507 duh

[OneWingedShark]

Maybe he used this mysterious map, which was based on ancient maps itself.

This is a mystery.

[zek157]

The new world wasn’t that new by 1500. I think the west likely got the Pacific Coast detail from the chinese fleets. For the east coast data the basques had been fishing cod in Nova Scotia/Grand Banks from the middle ages by that time. They provided Europe the salt cod for meatless holy days from about the 900’s.