I know voyages have been done for a long time. There weren't many undiscovered islands by then. There were a bunch of species found (my ancestor did a fair amount of that).
But how good was the temperature map?
How do they know what the water temperature was, across the whole Pacific, in 1877? At what resolution do they know it?
dlcarrier's answer (at https://news.ycombinator.com/item?id=48202959) says that they are using the data that they do have to derive the temperature, with "more precision than accuracy". They don't know that accurately, and what they do know, they don't know from "someone sticking a thermometer down there".
You may wish to have a look at this: https://en.wikipedia.org/wiki/European_and_American_voyages_...