Well, there is no such thing as a SH3 Globe.
The real world and the SH3 world have different shapes. The coordinates in the SH3 world (and SH4 and 5 I think) are basically the latitude and longitude axis on the globe. But the SH3 world is a flat rectangular plane, whereas the real world is a sphere (-like) shape. On the SH3 map the north and south pole regions are stretched out to the same size as the equator. (0 meters becomes 43200 km ...
) So you can't really compare both distances. Meridians on the SH3 map never come together at the poles, like they do in the real world. So horizontal distances on the SH3 map can NOT be compared to horizontal (East-West) distances in the real world. And compass headings don't match either in most cases.
That said, the game uses 120000 game-meters (or 120 'game kilometers') for each degree along the latitude and longitude axis. If you measure distances from a point to the Greenwich Meridian (vertical line through London) and to the equator (horizontal in the middle of the map), then divide those by 120, then you'll get the latitude and longitude degrees which you can use to locate on a real map, or Google Earth for example.
According to this Wiki page the length of the equator is around 40075 km. But according to the above scale, in the SH3 world it would be 43200km. From north to south pole would be 21600 km on the map, and in the real world it is about 40008 km.
The question is, what do you want to do with these distances and coordinates?
p.s. nautical miles (like for speed) in the game are just like the real value: 1 NM = 1852 meters