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Tim Dawson
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SemperFi, I don't really understand what relevance your last post has to what I posted. We have an excellent connection to our users and are well aware of how they're using our product. Some "straight" bits of airspace edge are rhumb lines and some are great circles, and our aeronautical database can cope with either.
Chris, I don't see there being a "new GC world". For most of our VFR customers, most of the time, RL is the best choice. You produce a PLOG, have one heading to fly, and that's it. If we were ever to default to GC, we lose the simple "one heading to fly" for a leg, and that simplicity is something that people are looking for.
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ckurz7000
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+x
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.
Thanks for the corrections. I, too, hate sloppy use of concepts and words. I tried to go back and correct my post but couldn't find how.... -- Chris.
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ckurz7000
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Tim makes a good point about how old-RL routes are going to fare in a potentially new GC world. Well, this is a transitory problem which goes away over time and not a principal conceptual problem. Each new route (planned as GC-route) would need to know that it is a GC route. When re-using an old RL-route, SD would know that there could be potential problems and can warn the pilot to check the route. It would then be converted to a new GC-route.
I don't see a problem there because the validity of a route can change because of many other reasons as well. Changing airspace can necessitate a change in routing, which is a far more likely scenario than a route changing from being RL to GC and consequently infringing on some airspace. If that should be the case, SD will have a warning issued under the warnings tab. From the perspective of the user nothing has changed: the old route infringes upon some hitherto unproblematic airspace which now has become problematic -- either by a change in airspace of the one-time coversion of the RL-route to a GC-route.
-- Chris.
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T67M
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+x
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?!?!  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.
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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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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.
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peter@luthaus.de
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+xThere are a few facts which maybe ought to be all together on one page: - 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.
- 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.
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peter@luthaus.de
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+xThanks 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
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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.
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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.
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