Various other factors may also dictate a flight path that is longer than the great circle path. A track system is used across the North Atlantic to efficently and safely manage the larger number of flights, even though this may result in a less efficient path for a given flight. There may also be a need to avoid certain areas, including areas too far from a suitable landing point (see the ETOPS section of the FAQ) or over unfriendly countries.
The Great Circle Mapper assumes the speed of sound is 574 knots (about 660.5 miles per hour or 1063 kilometers per hour), which is a close approximation for the lower stratospheric altitudes from 36,000 feet to well above 65,000 feet where transport jets usually fly, assuming a temperature of -70°F. More information is available from the following resources:
Perhaps a better answer would be if you could choose your own default. That feature is coming soon, but isn't quite ready yet.
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Karl L. Swartz.
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