FlightGear offers several specialist interfaces, one of which emits a stream of NMEA compliant position reports (the format used by GPS units) to serial port or UDP socket. This can be fed directly into one of those applications, which doesn't notice that this isn't coming from a real GPS, enabling the user to practice realistic tasks in the context of the simulated aircraft and all the realistic workload of piloting.
Data that is released into the public domain is generally of reduced quality, or out of date, or does not give widespread area coverage. The TerraGear scenery from such data is actually wrong, compared to the real world, but generally only in ways that are visually unobtrusive to the casual user.
These errors are much more visible in electronic navigation, such as needed for instrument flight, since the route tolerances are extremely tight. Navigating the simulated aircraft around imperfect scenery according to current Jeppesen (or NOS, etc) charts (or electronic databases) can be extremely frustrating and occasionally impossible when a piece of scenery is in the way.
To avoid the frustration, the Atlas project has developed software which automatically synthesizes aviation style charts from the actual scenery files and databases being used by FlightGear. These charts, while inaccurate to the real world and therefore useless for flight in an aircraft, are extremely accurate for the simulated world in which the FlightGear aircraft operate. Thus, it is often easier to make printouts from the Map program of the Atlas project.
The project also includes the namesake Atlas application. This can be used for browsing those maps and can also accept the GPS position reporting from FlightGear in order to display aircraft current location on a moving map display. This capability must be used selectively by the simulator pilot, since most small aircraft do not have built in map displays.
The Atlas moving map need not run on the same computer as the simulator, of course. It is especially valuable running on the instructor's console, where the pilot cannot see the picture, for gauging the student performance at assigned tasks.