Where the 2 lines meet. Basically every 2 pixels is a city (pretty much), it's just going round the map zoomed out and count the outer cities. Then it's just a case of loading the target city, go to world map, move directly north/south and then east/west to the end of the map. Then count cities on the outer edge to a point where you know from making the outline in the first place. Then you can plot it one line. Then you do the other. (N/S then E/W).
There is something funny about the map though, it never completely lines up when going around it. But it's not far off. Probably my counting.