I love treasure charts. I like doing them, in that weird searching sorta way, at least once i get good equipment- specifically the golden shovels with fortune and goldsight.
Despite this, there seems to be a bit of an issue with some areas- specifically Ravenna around the castle, shell island around the city and rarely even akursius keep with wrong directions.
i think in terms of where the “center” of the island is creates some issues with navigation, in Ravenna’s case, it’s sort of a wide island, while Akursius only has a few spots of actual surface that make it janky
I once had a chart right inside the courtyard
Also, try checking the few houses with grass layer on the terrace. I haven’t had charts there, but you can dig there, I wouldn’t be surprised
There are a few houses that have a terrace on top floor with a tiny patch of grass. You can dig there for a tiny amount of galleons, but no idea if charts can spawn.
Im fairly confident they cant spawn there, since ive yet to get a chart leading to super small patches of ground/sand. Im pretty sure a patch of ground/sand has to be of a minimum size before charts can spawn there