entering the whirlpool

You only need a map like that if your persona's "early period" (from my point of view). The Portuguese were obviously far ahead of the rest of the world (or something like that... sarcasm is lost here) as you can see by the detail in their maps here (http://gallery.sjsu.edu/cartography/maps/index.html). :) Knowing where to point your ship when you're going across big bodies of water helps to not get lost!