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Great Circles


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rg
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Tim Dawson - 5/16/2019 9:56:15 AM
I wish those people who were delighted with the temporary ability to switch to great circles had actually told us so, or given us some feedback, or indeed acknowledged the feature in any way. As it was, the only feedback we got was negative, from those who considered that it added unnecessary complexity to a product that people value for its UI simplicity.

It can only ever be a route-level option in SkyDemon, not a general one, because in the latter case all the routes ever planned and saved by people in the past would suddenly become great circles upon loading, therefore changing their course over the ground.

Would the reverse also be true?  if you have a great circle route saved and you switch back to rhumb line in settings and open the GC route it displays and saves as a rhumb line?   Doesn't that work OK as a user setting doesn't it so you're one way or the other?

Cat Burton
Cat Burton
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Why would we ever even need rhumb lines?

Even the CAA chart is a Lambert and therefore every line we draw is a great circle, or near enough.

Long east/west tracks done the paper way, we used to teach to measure mid track.

Cat Burton
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Tim Dawson - 5/16/2019 9:56:15 AM
I wish those people who were delighted with the temporary ability to switch to great circles had actually told us so, or given us some feedback, or indeed acknowledged the feature in any way. As it was, the only feedback we got was negative.

This is the problem of relying on feedback. It is in most people's nature to complain when something is wrong, but not to compliment when something is right. I'm sure you get usage data about which features are used and which aren't directly from the app, but if an option isn't present, you don't even get that.

WRT the Great Circle option in a beta tonks ago, I used it, I loved it, and I told you so.

Tim Dawson
Tim Dawson
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Cat, when you measure a track off a chart, it's a rhumb line. You fly a constant track (or aim to) for the length of the leg. Great circles don't work like that.
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The line on a CAA/NATS chart is a Great Circle. The line on a Skydemon chart is a Rhumb Line.

The heading in a PPL PLog is a Rhumb heading, but is rarely flown accurately, and is constantly being corrected, therefore small differences between RL and GC aren't relevant.

The track flown super accurately by an autopilot in Nav mode is a Great Circle, and isn't where Skydemon drew its line. The difference of half a mile or more between planning and flying can result in an infringement.

ArnaudC
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I put my vote for GC too.
I can live with my old routes becoming different because they were planned with RL, I always do some level or re-planning when re-using old routes anyway.

Cat Burton
Cat Burton
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No, Tim. A line on the paper chart is very definitely a great circle. If you measure a long east west track at the ends, you’ll see the tracks are not reciprocal, but differ by convergence.

That’s why we teach to measure the track at around mid point. To get an approximation of the rhumb line, for ease of flying. But the line remains a great circle.

Long haul flights would, typically, be given the initial true track and the mean magnetic track, but the FMS would happily direct the flight along the great circle.

Likewise, radio nav can only follow great circles (and the track should be measured at the VOR, for that reason, and for the local variation to which the VOR is offset. When tracking a long east west radial, the radial uses convergence and variation at the station and the aircraft will experience an apparent drift of convergence and the difference in variation at the start, gradually reducing as departure decreases.

Cat Burton
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Edited 5/29/2019 10:33:20 AM by Cat Burton
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