Garmin basemaps are the simplest maps available on a Garmin unit. They are included with a unit from the time of purchase. They typically only have one type of data that is specifically suited to the purpose of the GPS.
Basemaps typically correspond to the type of unit that they come with. Marine GPS units will have basemaps with detailed waterways but no roads. Aviation GPS units may have major roads but lack detail.
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Most Garmin units ship with a single basemap for the area in which they are sold. North American GPS units may not display a map while in Europe if they are only loaded with a North American basemap.
Only the iQue line of products can change their basemap. All other Garmin units always have the same basemap. You can add additional maps using Garmin MapSource or by purchasing add-on cards, but the original basemap will always be underneath them.
Basemaps for Garmin mobile units typically have major geographical features, large cities and towns, airports, political boundaries and major highways. Additional maps are often included to supplement the basemap with more detailed geographical, political and travel data.