Several of these units such as the GPS III+, GPS V, GPS MAP 76, GPS MAP 76S, and the eTrex Vista and Legend, have what is called "a built-in basemap". The benefits of a basemap are as follows:

Intuitive orientation. Simply turn the unit on, and upon acquisition of satellites, your location will be shown in the center of a map. Pick up the unit and start walking, and the map begins to move. You can see immediately what is happening, even if you know nothing about using the GPS.

Waypoint entering. With a basic GPS there are only two ways to enter a waypoint - entering the latitude and longitude of it (not always known), and actually going to the place and marking it (impractical on a first-time trip). A detailed basemap allows an additional method of slewing a cursor to a point and then marking it there. You can zoom in or out as necessary. For example, zoom out, slew across to London. Read the distance of your proposed trip. Zoom in and find the area of your hotel (the exact street if you have uploaded a London MapSource CD). Mark it now for help later in finding it upon arrival.

Interstate road trips. The permanent basemap contains lakes, rivers, cities, interstates, highways, railroads, coastlines and interstate exit information (USA only). Each exit shows lodging, gas stations, restaurants, hospitals and other facilities by name at the exit. Slew ahead to find the next Pizza Hut or Burger King.

Airline flights. Most airlines do not ban the use of a GPS in flight. If you have a window seat, plug in a low profile external antenna, and place it between the window and its partially drawn window shade. Watch the highways and cities pass beneath you on domestic flights. Watch the Aleutian Islands, eastern Russia and Japan come into sight as you fly to Tokyo; all the while having altitude, heading, ground speed, distance to go, and ETA displayed just as if you were in the cockpit.

MapSource CDs. There are a wide array of these CDs available to upload into GPSs that have basemaps.