Here’s my logic:

Luke finds an ancient map of all the Jedi Temples with a line showing their migration, keeps it a secret because look what happened to Jedha. After Kylo Ren betrays him, he goes into hiding by separating the map, giving half to R2 and half to Max Von Sydow, and follows the trail of the migration of the Jedi back to Space Ireland.

So it’s not that the map fills in a section of uncharted space, it could very well be charted, but imagine that in your example that its more like finding a guy standing between Utah and Kansas and you’re looking with the naked eye from Mars. As the scholar says:

“Space is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, but that’s just peanuts to space.”