So, when you travel the Grand Line normal navigation methods don't work because of the chaotic nature of place. Compasses just spin randomly, the weather can change without any warning, currents can make a ship change course without the crew even realizing etc.
For this reason, pirates who attempt traveling the Line rely on a Log Pose, a device that always point to a specific "next" island on a specific route. After crossing the Red Line again and entering the "New World" the Log Pose is then upgraded to handle the even worse conditions: we go from a single needle to three (because some island may just "decide" to become unavailable "because reason").
If you keep following the Log Pose you will eventually reach a "Final Island": Lodestar.
And then... your Log Pose stop working. There is no "next" island to point to, so the device you relied up so far suddenly becomes a decorative piece of furniture.
After you reached Lodestar, you are supposed to discover about the existence of the four Road Poneglyph (assuming a random guy on Zou didn't spoil you before). Using the four Road Poneglyph you are supposed to locate four points on the map...
and Laugh Tale is supposed to be found at the intersection of those four points.
So basically, Roger crew:
- followed the Log Pose up to Lodestar.
- somehow managed to "go back" from Lodestar to the previous islands where the four Road Poneglyph were located (without any way to know in advance that they would need to return there).
- used the info from the four Road Poneglyph to find Laugh Tale.
- somehow managed to reach an arbitrary "island" in the middle of nowhere that no existing Log Pose points to in a place where we have been told so far the only way to reach some point is to have something that points there.
Let's assume that Roger was able to "backtrack" in the Grand Line using multiple vibre cards and Eternal Pose that the crew strategically kept on hand during the journey (btw good luck crossing the red line without a Eternal Pose for Fishman Island, apparently that is the only safe place to travel under the mountains) IF the "Laugh Tale is found at the intersection of the four places indicated by the Road Poneglyph" is to be taken literally as the picture above implies (picture actually taken from the show) this means that to reach Laugh Tale you have to reach an arbitrary set of coordinates in the sea, without any modern GPS systemm, without a working compass, in a sea where apparently you can even be sure you are going in a straight line and where star based navigation just doesn't seem to work "good enough" to be usable (for example during the Alabasta arc Chopper mentions he couldn't see any star at night from his home island because of the permanent winter weather conditions, and Enies Lobby is literally stuck in an "eternal day").
You see where this is going.
How can anyone be able to reach 47°9′S 126°43′W under such conditions? And to be fair... how did all the pirates that "gave up" backtrack the line to return to their home sea (see: Don Krieg, Zeff) - where they just crazy prepared and kept an Eternal Pose for all the mayor island during their journey onward (this would make Nami suddenly look like a pathetic newbie since she is never shown planning for this... it is "forward or die" for the Straw Hats...). Either the inability to reach a set of specified coordinates in the Grand Line is greatly exaggerated or it should be impossible to reach any point you don't have a Log Pose pointing to.
Am I missing some obvious caveat that could explain this? Are my initial assumptions actually wrong? Or should we just accept that this point has no actual explanation right now and may become relevant in the future?
At the start of the Grand Line we are explicitly told that regular compasses don't work and implicitly that celestial / star based navigation doesn't work either because of the extreme chaotic conditions of the place (notice: there is even a place - Enies Lobby - where it is always day) and you have to use Log Pose that points to specific places to get around. Later, under the same restrictions, we are told that to reach Laugh Tale you have to find out its location and then travel to those coordinates without anything that points there.
Is there any indication of how that is possible so far in the plot? How can you reach any point even with a map if we are told that the Log Pose is the only reliable way to know which direction you are going since traditional instruments can't work (sextants, compasses, celestial navigation etc).