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Great Circles


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peter@luthaus.de
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As I started this thread and saw there is another reply: I see still the original points valid and there needs to be no choice between RL and GC, just use GC and there would be no discussion. The arguments are all given before. The option to select between the two would only be nice for training purposes, in real flying RL is not needed. I still find it a pity that SkyDemon behaves against all other industry standard in this respect.

Tim Dawson
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Peter, this thread has made it very apparent that the vast majority of PPL flying is conducted using a PLOG with rhumb lines, i.e. a leg with a constant heading to fly. It has also made it apparent (by its duration) that the vast majority of customers are happy with the way things are.

However, we still make efforts to accommodate your original request in some way.

Please try to see this from the perspective of other pilots‌‌.‌‌
ckurz7000
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There are a few facts which maybe ought to be all together on one page:

  1. The shortest course between to points on a sphere is a GC route. A RL is a course of constant heading between two points, which will always be longer than the corresponding GC course.
  2. To follow a GC track, one has to fly a constantly changing heading, beginning to end. This brings up conceptual problems in how to represent the leg on the PLOG, since you don't have one single heading describing it.
  3. For the vast majority of pilots there will not be a perceptible difference between RL and GC courses.‌ Because the difference between them only becomes apparent on long (> 500-1000 miles) mostly east-westerly legs.
  4. The bigger problem arises when you plan your flight as a RL (e.g., on SD) and fly it as a GC (e.g., a Garmin GPS) or vice versa. In this case you could potentially infringe on some airspace which looked like you would clear it during planning. This also is only a factor on long, predominantly east-westerly legs.
  5. Apparently it is not a conceptual problem for SD to incorporate either RL (which it does at present) or GC (which it did to some extent in the past). The only question is how to integrate RL and GC into the user interface in a consistent and intuitive way which doesn't confuse those 99% of GA VFR pilots (and SD customers) who are not concerned with this issue.
There are two solutions which I find acceptable with a slight preference for the second one:
  1. Kee‌p everything as it is and, b‌y default, plan and navigate a RL course (so no perceptible change to 99% of users). However, if at any point the RL course deviates by more than a given amount (i.e. 0.5 nm) from the GC course, or if the RL course is more than a set amount (i.e., 5 nm) longer than the GC course, issue a warning under the "Warnings" tab. Alongside with the warning, offer the user the option to show the GC course (maybe as a faint line) so that the leg can be conveniently split into several RL sections, thereby approximating the GC course.
  2. Alternatively‌‌, you could switch over to GC navigation and plot everything as a GC course. The switch would not be noticed by 99% of SD customers anyway. In the PLOG use the average heading over the course for display and computational purposes. You would not need to issue a warning since navigation would always be by GC. There will be an ever so slight error in the PLOG, but hardly noticable since one has to correct for wind anyway which is only a forecast guess. And navigation is done by "following the line", so steering a changing heading isn't an issue in practice. Furthermore, no need to introduce the issue of GC vs RL to unsuspecting pilots who don't know what to do with this. The interface presented to the pilot would be consistent, correct and intuitive.
Greetings, -- Chris.

Edited 1/18/2018 12:00:02 PM by ckurz7000
Tim Dawson
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Thanks Chris, that's an excellent summary of the state so far.

The main problem with suggestion number 2 is that there is a library out there of tens of thousands of routes planned and saved by our users, and loading any of those routes going forward would result in a different track over the ground than when they were saved. Thus somebody might load a saved route deemed safe, and inadvertently infringe or simply go somewhere they didn't plan to. ‌‌

T67M
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That is indeed an excellent summary of the discussion thus far, the only addition I would make is to highlight that PPL navigation is neither GC nor RL but some kind of unholy combination of the two.

I‌ share Tim's concern with solution ‌‌2, but solution 1 is very similar to the one I proposed earlier, the only difference being that the user must spot the message in the warnings tab and manually enable the GC track display on a per route basis. I would prefer a per user setting (default off to preserve the current modus operandi) which enables the display of the GC track automatically when it is sensible to show it. As an instructor, I would use such a feature when teaching people navigation so that they are aware of the different compromises between GC, RL and PPL navigation, and when to use each.

For‌ info,‌ the difference between GC and RL navigation is potentially significant on a flights as short as Sandown to Headcorn.
peter@luthaus.de
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Tim Dawson - 1/18/2018 12:45:42 PM
Thanks Chris, that's an excellent summary of the state so far.

The main problem with suggestion number 2 is that there is a library out there of tens of thousands of routes planned and saved by our users, and loading any of those routes going forward would result in a different track over the ground than when they were saved. Thus somebody might load a saved route deemed safe, and inadvertently infringe or simply go somewhere they didn't plan to. ‌‌


Hello Tim,
then I would suggest to issue a warning concerning the saved routes. What is the greater danger for infringement: Planning RL and flying GC because the built in GPS does so or reloading saved routes (with the dominating majority so short there is no danger). The longer you wait, the more routes are saved and this problem increases. I predict at some point also SkyDemon will change to GC, because it is a nuisance to differ from built in units and there is even a risk of airspace infringements. It is only a matter of time. A co-existance of GC and RL would be confusing to a lot I guess.
Peter

peter@luthaus.de
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ckurz7000 - 1/18/2018 12:00:02 PM
There are a few facts which maybe ought to be all together on one page:

  1. The shortest course between to points on a sphere is a GC route. A RL is a course of constant heading between two points, which will always be longer than the corresponding GC course.
  2. To follow a GC track, one has to fly a constantly changing heading, beginning to end. This brings up conceptual problems in how to represent the leg on the PLOG, since you don't have one single heading describing it.

Besides being correct with most of your statements, I just can't see the terms heading and course mixed, so sorry I have to correct.
‌So point 1:
"A RL is a constant course between....." not heading. As you write yourself, the heading changes with wind and (academically for us) variation.
And point 2:
"... has to fly a constantly changing course" not heading (the heading is of couse likely to change too). Or as Garmin calls it DTK (desired track).
So  the PLOG has anyway the problem of a changing heading an building an average, because the wind can change. And it is a common practice to use the averages in flight logs.
‌‌

Edited 1/18/2018 1:30:10 PM by peter@luthaus.de
Tim Dawson
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Peter, I think you grossly overestimate how many people use built-in GPS to navigate with. Most people have only SkyDemon, and use SkyDemon to navigate with. That's why this has historically never been much of a problem.
Tim Dawson
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The best solution I can come up with at the moment is to have a per-route setting somewhere. Then an option elsewhere which sets what the default is for new routes. And we'd also have to highlight in the PLOG somewhere (probably the Trk columns) if the value being displayed was the initial track for a GC leg, or the complete track for a RL leg.
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SemperFi - 1/19/2018 2:28:22 PM

Are the airspace borders in Skydemon also constructed from RL instead of GC? If yes, o-O!

That's a very interesting question - the wording for airspace here in the UK is along the lines of:
511258N 0001129W
t‌hence a straight line to 511200N 0000341E
‌thence clockwise by the arc of a circle radius 10 nm centered on 510853N 0001125W to 510550N 0000342E
t‌hence a straight line to ‌510240N 0001923W
‌thence clockwise by the arc of a circle radius 8 nm centered on 510853N 0001125W to 511118N 0002332W
‌t‌hence a straight line to 511258N 0001129W

T‌his gives no indication as to whether the straight segments are rhumb lines or great-circle lines. The curved sections are even more interesting - are these rhumb-circles or great-circle-circles, or great-spirals?!?! Wink

‌‌The good news is that a visual comparison of the SkyDemon chart and the official CAA/NATS paper chart shows that all the lines are in exactly the same place relative to plotted ground features, including the mid-points of long, straight, east-west airspace boundaries.

GO

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