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This is a screenshot of a Facebook post discussing a flight between Sydney and Perth. The line bends to the North, but shouldn't it bend to the south if its following a great circle? Or, do pilots on this route perhaps prefer to avoid flying over the ocean?

screen shot of flight between Sydney and Perth

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    I'm gonna bet that dotted line you see is explicitly a user experience feature and has nothing to do with the actual plane route. – fredsbend Oct 29 '18 at 21:29
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    Facebook draws the same dotted curved lines if you tell it you are travelling to the next town over, 10 miles down the road. – Chris Clayton Oct 30 '18 at 15:20
  • The most direct route between two points is a straight line, not some "great circle" mumbojumbo. Is your map a Mercator Projection, by chance? That is a very flawed map. – Harper Oct 30 '18 at 16:02
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FlightAware has a handy IFR Route Analyzer that will supply some of the most common routes from flight plans between two airports. We can use it for Sydney to Perth.

The most common route as of this writing (it varies from day to day depending on what flight plans airlines file) is DCT KADOM H44 AD Q33 ESP Q158 BEVLY, which, if we plot it, does indeed bend to the south (the other common routes are pretty similar):

enter image description here

That's pretty close to the great circle route, though actual flight paths are adjusted for winds, weather, and air traffic control considerations.

Facebook's display is not really meant to be particularly representative of actual flight routes, or perhaps they're hemispherist and failed to consider how great circle routes work in the southern hemisphere.

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    My money would be on Facebook (once again) not fully considering how conditions might differ outside of the USA. – Michael Seifert Oct 29 '18 at 19:57
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    @Michael My money would be on that dotted line symbolizing a plane lifting in the air and flying to its destination. It's a user experience feature, not a representative example of where the plane actually flies. – fredsbend Oct 29 '18 at 21:30
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    DCT KADOM H44 AD Q33 ESP Q158 BEVLY – I’m not familiar with this notation. What is it called and where can I learn more? – sjy Oct 30 '18 at 9:39
  • @sjy it might look cryptic, but there's not actually much to learn about the notation (and I'm not aware of any particular name for it) - it's just a list of the navigational points passed and the airways followed. The name scheme does give some information about the points/airways, though unfortunately I can't seem to find a good reference for it. In short, though, 2/3 letter names indicate radio navigational aids, five letter names are waypoints defined by position relative to multiple aids (or, now GPS is common, just by latitude & longitude), and the ones with numbers are airways. – Chris H Oct 30 '18 at 11:13
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    @sjy I don't know if that scheme is necessarily worldwide, but afaik at least US/Europe do it that way and this route in Australia seems to fit it too. So this route heads, after departing, to the waypoint KADOM, then follows the airway H44 to the navigational aid AD, from where it follows the airway Q33 to navigational aid ESP, then the airway Q158 to the fix BEVLY. – Chris H Oct 30 '18 at 11:15
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It's just Facebook misrepresenting how the plane flies. Google Maps does the same thing:

enter image description here

Rome2Rio shows approximately the right path:

enter image description here

Note that Facebook and Google are both companies that were founded in the USA, while R2R was originally an Australian company. This difference in location probably had an impact on how well they display commercial flight routing in the Southern Hemisphere.

  • Tbh it's more probable that the arrow has to do with how they chose to represent the plane going up. Also on the southern hemisphere planes go “up“. – Hakaishin Oct 30 '18 at 17:21
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    @Hakaishin what do you mean "up"? Great circle navigation on a sphere in the southern hemisphere appears like a southward curve on a flat map. – Nzall Oct 30 '18 at 21:14
  • I can confirm that this is more or less the route we took when flying from Sydney to Perth, as we traversed the peninsula near Port Lincoln whereas the FlightAware sends the plane below it. – dlanod Nov 11 '18 at 22:06
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The arcing line is nothing more than a stylistic riff to show that you are "hopping" from point to point. This style predates pre-dates consumers having ready access to flight, e.g. It would be seen in pre-war films in those interstitial "cards", illustrating that the story was moving on. The hop is always upward, to indicate "up" or "passing over" (that part of the story).

The thing today, where every travel map shows a fastidious capture of actual surface routing (for consumers for whom it makes no difference), is a modern craze owing to map apps and nav's.

In this case, the stylistic riff is being used to say, flat-out, this isn't your actual surface route. If they had drawn a plain straight line, it could be mistaken for that.

The most direct route is always a straight line, even in the Southern Hemisphere. There's no such thing as a "Great Circle route" that is faster by flying a curve. Remember, we are deliberately looking at flat projections of the surface of a sphere. We are aiming to ignore the one curve you must follow, the solely vertical curve of following the surface of a sphere.


At this point we must discuss the Mercator Projection, a particularly defective kind of map where longitude and latitude lines are forced to be square. You can spot it instantly, when state and province borders are mis-shown intersecting square without bends: Ask any mapmaker, that isn't how things are! This fault also makes Greenland appear bigger than Australia. Above, Facebook and Rome2Rio get it wrong, using this shabby projection. Rome2Rio shows the actual surface route, but since they have bent the map to make curved lines square, the route bent with it.

They call this mistake the "Great Circle Route". Mmmmkay.

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    Any flat map will turn some "great circle routes" (shortest route possible on the surface) into curved lines. The mercator is not "particularly defective", it is just one of many possible compromises in mapping the earth to a flat map. – Peter Green Oct 30 '18 at 17:17
  • @PeterGreen sure, but with other projections including the "here is what the globe actually looks like if you shot a photo from 10 light years away" projection, if you center the map over the expected flight path, the line will be pretty darn straight. – Harper Oct 30 '18 at 17:20

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