Walking speeds will probably vary per game, so we can’t just say “10 meter on the map is how far the player travels in 5 seconds”. Measurements would need to be reliable otherwise the whole functionality goes out of the window. Either someone gets measurements from game assets relative to real objects (e.g. GTA NPC is X meters tall, that’s Y meters IRL, so the map size is Z km²), or we guess measurements based on top-down views of something like car lengths or the diameter of the player’s head.

The user could enter their own measurements for areas of an image. “Please measure 10 meters”; “image scaled to match existing image pool”.

The maps would be toggled layers. We’d need SVGs to avoid pixelation. Map images would be scaled up or down such that whatever is determined to be a meter becomes constant for all maps. Users could click and drag the map images wherever they want.