The uncharted island map is a map which can be purchased for 800,000 coins from the travelling merchant, or found randomly while fishing at the Deep Sea Fishing hub. The map leads to an island in the Uncharted Isles with 4-6 high-level special resources, as opposed to islands with 3-5 high-level special resources generated from long voyages or regular uncharted island maps.
A map purchased or received from fishing may rarely come with an additional red uncharted island map, which grants the player access to an island that contains Tavia's fishing rod.
Reading the map prompts the player to say "If I take this to my quartermaster in the Wushanko Isles, I can visit this island."
For an exhaustive list of all known sources for this item, see here
This information has been compiled as part of the update history project. Some updates may not be included - see here for how to help out!