
This image is old and does not reflect the current world. It also doesn't have exactly the same coordinate system, but I didn't realize how unwieldy it would have been until I thought of making NNNWWW.
Source: viewtopic.php?f=7&t=4053 [he edited his post to show my image]
Back in ye olde I-be-mappin' with the ingame toolin' times, I devised the NyEx system.
Seemed kind of obvious, and has been mentioned here, and I think was actually discussed and previously decided in IRC, although I may have been the only one to remember that conversation.
N0E0 would be the default convention. That is, when there is a 0, N or E are used. If positive in the southern or eastern directions, S and W are used. This corresponds to a standard grid system where X+ is right and Y+ is up.
N2W1
S2E2
N0E1
Can be further expanded, such as N0E0[A/B/C/D] to signify region, or use roman numerals to refer to quadrants instead, then you can specify local maps within the region and 100x100 area if you like.
Like S2E2-IV-A2e4
Looks like a filename but given the size people tend to consider their area, region will probably be as low as you'll specify most of the time. I don't know why the A1 system came about when they started mapping this world. I guess Bigfish is silly?