User blog comment:Kaido King of the Beasts/Chapter 818: Stepping Stones to One Piece/@comment-24442815-20160303163252/@comment-10543621-20160304013837

It sounds like the coordinates are all islands. Which brings me to the other question of how Raftel can be considered the 'final island' if there are gonna be two more 'after' it.....

Well, in any case, even if they aren't islands, the main thing is just having the coordinates wouldn't work unless you have a skilled cartographer, who personally travels to said locations in order to draw an accurate map to point out at the final location I guess.....

Or you know... Plot dictates that the information on the poneglyph provides more than just coordinates, but directions that are followable by skilled cartographers. Of course, Nami is like the cartography goddess sooo....