Map pieces can be found as drops from various land and ship-boarding enemies all accross the world.
In order to put them together, go to the Treasure Hunter in Barcelona (City).
Make sure you have the EXACT amount of map peices you wish to use for the map, because the Treasure Hunter will take ALL of the pieces in your personal inventory. I suggest leaving extra or unused peices on your ship or in your warehouse.
Level 1 Treasure Map - 6 Treasure Map Fragments
Level 2 Treasure Map - 12 Treasure Map Fragments
Level 3 Treasure Map - 24 Treasure Map Fragments
Level 4 Treasure Map - 50 Treasure Map Fragments
Level 5 Treasure Map - 80 Treasure Map Fragments
"Only Sea Treasure Maps may be used to create a level 4 Treasure."
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