Coordinates: How to geographically position the character's position on a map?

Beyond the different coordinate systems (x / y, …), imagine that you draw the map of your world in a bitmap, png, gif or any image.

Then you have your world and you would like your players / character (PC) to travel through its different parts (lands, regions, etc.).

How can I simply locate in which part (earth, region, …) of the world a PC enters or exits? Detect when a limit is crossed?

What is the simplest way?
What are my options ?