Coordinate of map origin, in the format given by clicking on a sextant in the game: (like 53'23"s, 0'0"w). If you get the format wrong, it'll do weird stuff. Not my problem. Default is map 9, Britannia's origin:

Coordinate you want converted to map coords, in the same format:

Be warned! The coordinate system from the sextants can only ever be a guideline. The maps can change the coordinates for specific areas of the map. In particular, if you go to 0x, 0y of map 9 (the main world map of Britannia) and use the sextant, it reports a position that is completely inconsistent with the rest of the map. Also, maps other than the main world map may have different scaling factors: even on map 9, it varies from 8.5 to 9.1 units per arc-minute ('), and the scaling factors for x and y seem to fluctuate independantly of eachother.

Given that, the best you can hope for is that you'll end up on dry land fairly near the coordinate you wanted. You should be able to walk, or at worst, fly there with little trouble.

Remember there are 60 minutes in a degree, and 10'3" is NOT the same as 10'30"!