I'm sure map makers already know this, but it came up in a discussion in SD general...

In game, the world map is rotated -45°

Whenever I create a new world, I screenshot the world map, rotate it -45° and drop it into this file I made that orients everything correctly:

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=765425241

Using this, the compass and the sun ingame correctly relate to the world map island locations.

When I make and add new maps to the world, I always put them in the slot just to the right of the start Island (this is map 3_2 in the world folder, btw)
In game (and according to my rotated map), that is the NE slot.

and sure enough, there is my custom map island to the NE of the start island in game:

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=765726963