Minecraft wiki entry for Map, associated text: Other players are displayed on the map only if they have a map in their inventory cloned from the one being looked at. When placing a map into an item frame, the map displays with a green pointer shown at the location of the item frame. This is to help the player see where they are in relation to the area that the map is showing. If the player leaves a map in an item frame and views a clone of it, the green pointer remains in the spot of the framed copy. This can be used to set up waypoints. Unexplored areas are transparent, making the item frame visible.