Define a surjective function from the game coordinate system to the world map.
Suppose we have a game world where all points are between (0, 0) and (100, 100). We can then create a world map that is 10 pixels by 10 pixels and a function f(P) = (P_x / 10, P_y / 10) to map from the game world to the world map.
Let's suppose that we want more detail on our world map so instead of a single 10x10 world map, we create 100 10x10 world maps. First we determine which world map to use, WM(P) = (P_x MOD 10, P_y MOD 10), then we determine where on that world map we are, f(P) = (P_x - 10 * (P_x MOD 10), P_y - 10 * (P_y MOD 10)).
This can be taken a step further (see World of Warcraft) where the game world is divided into larger pieces, zones, and each zone has its own world map which can be broken into pieces like the previous example.
Ultimately this is a design decision with no technical limitations imposed as part of the questions. As best method depends on your requirements, I encourage you to explore how other games have addressed this on a conceptual level and build from that.