The name Costa Alta occurs on some early 16th c. maps of North America, like Waldseemüller's (first plate, fifth name from the top), and Schöner's. I understand this now means "high coast", as in mountaineous, and am wondering if there are any other possible interpretations? The only mountains on the coast are some way up in Canada, which is much further north than those maps seem to show.
Edit: the early maps, such as Cantino's (1502), Waldseemüller's (1516), and Schöner's (1520) end abruptly somewhere in the middle of the current U.S., and were presumably based on the early Spanish explorations starting in the Caribbean. But the name also occurs on later maps, like Vopel's (1540) that show the whole coast.