For short range air navigation the most general method of determining the distance between two points, and the course to be steered from one to the other, is by means of a grid system of coordinates based upon some map projection. Problems must arise, however, in routing aircraft between Traffic Control centres in adjacent regions which employ different grid reference systems, even though the airway markers are identified by their latitude and longitude. A difficulty which may result in steering-angle errors arises when these markers, which are generally VOR stations, are referenced to the local magnetic field rather than the geographic pole.