Navigating in the grand line

[taiwan luthiers]

I just thought of something:

Due to the weird magnetic properties of grand line islands normal compass doesn't work so they have to use a log posse or eternal posse that points to a specific island. It takes time for the log posse to set so I was wondering why can't they just use an eternal posse, from any island (as long as they know which direction that island is at) and sort of calculate where the other islands are and basically use an eternal posse like a normal compass?

But then again given how they have technologies that seems out of time (like refrigerators) someone would have thought of launching GPS satellites making compass/log/eternal posse obsolete…

[Ichnob]

Well, they don't know the actual direction of where the next island is located. And they only have 1 (maybe 2) Eternal Log Poses: Alabasta, and Maybe Longring Longland (forgot if they gave that back after the Donut Race). As you get further and further away from the starting point, there's a huge margin of error trying to calculate where an island would be from an Eternal Pose of an island you already passed. It could theoretically work if they had multiple Log Poses for upcoming islands, but I doubt there's one of a New World island that is easily obtainable in the first half of the Grand Line.

[threedayslater]

If they're keeping every eternal pose they've come across, they would have three: Alabasta, Jaya, and Long Ring. And yeah, those really wouldn't be much help in the new world at all.