Why flight routes curve on a map
Updated
A flight from New York to Hong Kong does not head west over the Pacific. Its shortest path runs north over Canada, passes within about 6 degrees of the North Pole, and comes back down over Siberia and China. On a flat world map that looks like a detour. On a globe it is the most direct line there is.
What a great circle is
A great circle is any circle drawn on a sphere whose center is also the center of the sphere. The equator is one. So is any line of longitude joined with the one opposite it. Between two points that are not directly opposite each other, exactly one great circle passes through both, and the shorter arc of that circle is the shortest path across the surface.
Why the shortest path looks bent
Most web maps, including AvMap’s flat map, use the Web Mercator projection. Mercator keeps local shapes and angles correct, which is why streets meet at the right angles, but it stretches the map more and more toward the poles. Greenland looks about the size of Africa even though Africa is roughly 14 times larger.
A straight line on a Mercator map is a rhumb line: a path that keeps the same compass heading the whole way. Rhumb lines are easy to steer, but they are longer than great circles. The difference is small for short flights and for flights that run mostly north–south. It grows quickly on long east–west flights at high latitudes.
| Route | Great circle | Constant heading | Extra distance |
|---|---|---|---|
| New York JFK – London LHR | 2,991 nm | 3,109 nm | 118 nm (4%) |
| Sydney SYD – Dallas DFW | 7,456 nm | 7,486 nm | 30 nm (0.4%) |
| New York JFK – Hong Kong HKG | 7,003 nm | 8,826 nm | 1,823 nm (26%) |
| San Francisco SFO – Dubai DXB | 7,030 nm | 9,105 nm | 2,075 nm (30%) |
Calculated from AvMap’s airport coordinates on a sphere with a radius of 6,371.0088 km. The great circle from San Francisco to Dubai reaches 88.2°N, less than 200 nautical miles from the North Pole.
How AvMap calculates distance
AvMap uses the haversine formula, which gives the great-circle distance between two latitude and longitude pairs on a sphere. It uses the mean Earth radius of 6,371.0088 km. Airport positions come from the OurAirports dataset, which AvMap refreshes every week.
The Earth is slightly flattened at the poles, so a spherical model can differ from a full ellipsoid calculation by up to about 0.5%. For JFK to LHR that is at most about 15 nautical miles. AvMap shows results in nautical miles, statute miles, or kilometers. One nautical mile is exactly 1.852 km.
Why real flights fly farther
Airlines plan close to the great circle, but rarely on it. The distance flown is usually longer for several reasons:
- Winds. Over the North Atlantic, oceanic tracks are published daily to follow or avoid the jet stream. An eastbound flight may go hundreds of miles off the great circle to pick up a tailwind and still arrive sooner.
- Airways and air traffic control. Departures, arrivals, and busy airspace follow set routes and sequencing.
- Closed or restricted airspace. Conflicts and airspace closures can add hours to some routes.
- Diversion rules. Twin-engine aircraft on long overwater flights must stay within an approved flying time of a suitable diversion airport.
For that reason, describe AvMap’s figure as the great-circle or direct distance, not the distance flown. AvMap is for visualization and is not a flight planning tool.
Try it
Open JFK-HKG on the globe to see the polar route, then switch to the flat map to see the same path curve. To compare several long-haul routes at once, try JFK-HKG, SFO-DXB, SYD-DFW.