This map, or series of maps, is the lowest level maps in a GPS device. This map is what you will base your waypoints and additional street maps off of. All GPS devices come with a standard basemap that covers highly populated regions, cities and other areas.
Some units come with permanently built-in basemaps, which typically include coverage of oceans, rivers, and lakes; principal cities, smaller cities, and towns; interstates, highways, and local thoroughfares; and railroads, airports, and political boundaries. Basemaps are available in a variety of global coverage areas, depending on the user's needs.