Don’t have to have planes, only the GPS Satellites the plane is using to update INS system. Don’t have to miss by much to hit terra firma unexpectedly. Especially if the aircraft is in the weather.
This presuming that the pilot has done no preflight planning. There are min enroute altitudes, minimum obstacle clearance altitudes (MOCA). Additionally no pilot relies solely on GPS altitude readouts. If they do, they deserve death.