I’ve watched this movie three times, and the map still doesn’t make sense to me.
The map that Po retrieves from Jakku is a fragment of a larger map. Threepio says it doesn’t match any of the Republic’s records, so this is apparently uncharted space (at least as far as the republic is concerned).
Artoo has been in low power mode since Luke left. Later, Artoo revives and reveals he possesses the complete map of Luke’s journey, complete with a line showing which planets he’s been to. There’s a convenient blank spot in the map, in which the earlier map fragment fits.
This doesn’t make a lot of sense to me because
A - If the first fragment doesn’t match any known records, then at least the parts of Artoo’s map (if not the entire thing) surrounding that gap would also have to be uncharted. It’s like saying “we have a map of the United States, but there’s a mysterious gap between Utah and Kansas.”
B - So clearly somebody charted all this space, but we don’t know who. Possibly Leia’s agents, but it seems like they’d be relaying this information back to her on a regular basis. We know this can’t be the case because the first fragment doesn’t match their records. Maybe this will be explained in later movies?
C - If Artoo has been shut down since Luke left, how can he be in possession of a map tracking his journey? Furthermore, if he had the map this whole time, why didn’t he tell anyone? Sure, it wouldn’t give Luke’s precise location, but they would still be given a major clue. Going back to my US map analogy, you wouldn’t have Colorado mapped out, but you at least know he’d be between Utah and Kansas.