Great-circle route, route on the earth's surface based on the great circle, any circle that divides a sphere into equal halves. The great-circle route is always the shortest route between 2 points. It appears on a gnomonic projection map as a straight line; on flat maps it often appears as a longer curve. By the 1800s ship navigators set their compasses to follow lines called rhumb lines, which join selected points along the great-circle route. Airplanes follow the great-circle route by using a method called inertial guidance.