It's like this. The Discworld is flat. Therefore its cartographers don't have to worry about all that fiddly stuff like progressions, projections, and figuratively opening up the globe like the skin of an orange so as to best transfer its representation to a flat plane. No point: it's already on a flat plane and what you see is what you get. It was Discworld map-maker Jorkie Wilson, unwinding over a game of darts in his local pub one night, who had the inspiration for the grid co-ordinates in use today, by which a Discworld explorer may patiently work out whose jurisdiction he is in and by extension which local despot is likely to hang, execute, imprison or otherwise sacrifice him to the Gods for soiling his sacred turf.
Beginning at Cori Celesti, known in this scheme as The Bullseye, the Disc is divided up by eighteen concentric circles. After an abortive first attempt, it was decided these should be evenly spaced at 10º intervals. 0º is Cori Celesti; 180º is always The Rim, whatever direction you choose to travel in. This second division is catered for by having thirty-six radial lines - twenty, in the original conception - radiating out from Cori Celesti like evenly-spaced spokes on a wheel. Now all that was needed was to make one of these radial lines into an absolute reference, the master line of longitude and key meridian that all others would take their cue from. After much squabbling and arguing, it was decided that 0º longitude should pass through the middle of Ankh-Morpork and be called the Dimwell Meridian.(1) The means by which this goal was achieved are not clear but delegates to the International Geographical Congress, hosted by pure coincidence in Ankh-Morpork, were shown great hospitality by the city elders and the Guild of Seamstresses.
(1) Although Jorkie Wilson's original schema called the city Treble Seventeen.