Why do airplane routes look funny on world maps?
The shortest distance between two points is a straight line, right? If you want to fly, boat, bike, or drive, following a straight line saves time and money. When you look at aircraft routes on flight tracking sites, does it look like airliners take the long way home? Why do they fly those curvy routes across the oceans and continents?
Spherical Globes and Flat Maps
The earth, as we know, is a sphere (more accurately, a spheroid). Maps are flat. Imagine trying to cut and stretch a globe into a flat rectangle. Tough job! Cartographers need to distort the shapes of land masses in order to fit the entire planet on flat paper. The maps pilots use, like road maps, only display a small portion of the planet so the distortion is minimal. This distortion is what causes straight lines to look curved on maps.
What’s a great circle route?
The shortest route between any two points on the earth’s surface is called a great circle. Although air routes look curved on flat maps, airliners do try to fly straight lines between cities. Exact routes vary due to winds, flight rules, and political borders (we don’t fly over certain countries). There are trig formulas for doing great circle calculations, but it’s more fun to find them with a globe (or other spheroid, like a basketball) and a piece of string.
Pick two cities on the globe, like Tokyo and Los Angeles. Stretch the string between the two cities and wiggle it around until you’ve found the shortest distance. That’s it! You’ve found the great circle route (and preferred airline or ship route) for those two cities. If you were to slice the earth along your great circle line, the earth would be cut into two equal halves. Try different cities around the planet. You might be surprised at the routes you discover. The above map shows a typical route between Tokyo and Los Angeles. If you try the string trick, you’ll see that the route displayed on the map is pretty close to where your string falls.
Polar Routes – Over The Top of The World!
Cathay Pacific’s ultra long-haul New York to Hong Kong flight looks really strange on a flat map. It appears to head north, turn west for several thousand miles then south to Hong Kong.
Is this really the route the airplane flies? You bet! The area near the top of the map is extremely distorted. The straight course represents the short portion of the flight that transits the north pole. Yep, this flight flies over the Arctic Circle near Santa’s Workshop. The total flight distance is 7000 nautical miles (nm). The segment over the pole (that looks really long on the flat map) is only 1200nm, or less than 3 hours of this 15 ½ hour flight. When you look at the route on a sphere, it makes more sense. Try stretching a string on a globe between New York and Hong Kong. Sure enough, the shortest distance passes close to the North Pole.
Polar trivia: Russia opened four cross-polar routes in 1998. Prior to this, polar flights were difficult to arrange due to poor communications in Siberia. Cathay Pacific flew the first cross-polar passenger flight in July of 1998 from New York to Hong Kong. This was also the first flight to land at Hong Kong’s new Chek Lap Kok Airport.
Great Circle Resources Need more knowledge? Look no further…
Great Circle Mapper: This online resource does all the work for you. Enter two cities and Great Circle Mapper displays the great circle route on the most appropriate style of map. Plenty of customization options for Map Geeks.
Wolfram MathWorld Great Circle page: If you want to work the formulas, Wolfram MathWorld is the place for you! I’m a product of the public education system, so I’ll stick with my beach ball and string… and my Flight Management System.
Wikipedia Great Circle Navigation Page: Nice overview of the great circle navigation and a few formulas to play with.