SkyDemon Forums

Great Circles

http://forums.skydemon.aero/Topic19428.aspx

By Stratajet - 1/18/2017 4:53:03 PM

T67M - 8/15/2016 7:07:20 PM
[quote]
Tim Dawson - 8/15/2016 2:10:24 PM
I took a look around inside our routing engine to remind myself of how we used to allow the switch between rhumb lines and great circles, and it turns out the architecture is all still there. For anyone interested, I have produced a beta build of SkyDemon Plan which treats all legs as great circles instead of rhumb lines. That doesn't mean we will necessarily ship something, but it would be nice to get some feedback on how it works.
Thanks Tim - the difference in the ground track on long legs is clearly noticeable, but the confusion of having the reverse leg not being a reciprocal heading is clearly apparent, and obviously the heading given on the PLog is only valid at the point of departure.

I like your idea of keeping the Rhumb Line as the default and using that for the PLog, but also drawing a (thinner) Great Circle line where the mid-point cross track difference is greater than a user configurable distance to guide the creation of additional waypoints in the leg if/when the user understands and wants Great Circle Navigation. I feel that the the default value for the cross-track threshold should be quite large so that existing users aren't confused by the changed behaviour.

I'd like to +1 the Great Circle line with heading as a Rhumb on the PLog. It's really frustrating having the GPS being better at getting me there more direct. It was a particular pain when I was in Greenland a few months ago.
By ckurz7000 - 1/24/2017 1:34:43 PM

Stratajet - 1/18/2017 4:53:03 PM
T67M - 8/15/2016 7:07:20 PM
[quote]
Tim Dawson - 8/15/2016 2:10:24 PM
I took a look around inside our routing engine to remind myself of how we used to allow the switch between rhumb lines and great circles, and it turns out the architecture is all still there. For anyone interested, I have produced a beta build of SkyDemon Plan which treats all legs as great circles instead of rhumb lines. That doesn't mean we will necessarily ship something, but it would be nice to get some feedback on how it works.
Thanks Tim - the difference in the ground track on long legs is clearly noticeable, but the confusion of having the reverse leg not being a reciprocal heading is clearly apparent, and obviously the heading given on the PLog is only valid at the point of departure.

I like your idea of keeping the Rhumb Line as the default and using that for the PLog, but also drawing a (thinner) Great Circle line where the mid-point cross track difference is greater than a user configurable distance to guide the creation of additional waypoints in the leg if/when the user understands and wants Great Circle Navigation. I feel that the the default value for the cross-track threshold should be quite large so that existing users aren't confused by the changed behaviour.

I'd like to +1 the Great Circle line with heading as a Rhumb on the PLog. It's really frustrating having the GPS being better at getting me there more direct. It was a particular pain when I was in Greenland a few months ago.


+1 for optionally showing a faint version of the GC course (if the deviation is significant) because it gives me the option to approximate it using several shorter RL segments.

-- Chris.‌‌
By peter@luthaus.de - 7/27/2016 12:01:14 PM

Hello,
suggested by Tim I post this request in the forum for discussion. I noted (it was already mentioned in the forum) that SkyDemon uses generally rhumb lines to connect waypoints. I find this unsatisfactory and annoying. Industry standard and the wanted way is great circles. There are many reasons why great circles should be used (compared to none using rhumblines):
1) Great circle is the standard for GPS navigation, so built in certified GPS use it and Skydemon is not consistent with the track actually flown even producing off track warnings.
2) For planning the total distance is not so different for short legs, but the off course deviation can be quite large. E.g planning from EDLW to EPWR the rhumb line goes through EDDP Delta, the great circle passes it completely.
3) Most pilots use Lambert projection charts approximating the earth's convergency for planning. So the straight lines drawn on paper will be close to great circles and it is unexpected that skydemon shows a different route.
4) Radials are great circles
5) Airways are great circles
As I find the rhumb line really a nuisance and also other pilots I know do so I suggest changing to great circles. I hope this could be implemented soon.
Peter

By greg - 1/24/2017 9:39:23 PM

+1.
‌Imagine you plan a transatlantic flight, this would be most useful!
By ckurz7000 - 1/29/2017 8:21:40 PM

Dou you really rely on a VFR only tool like SD (with probably no map coverage for a transatlantic route) to perform such a flight??????? And if you really perform such a flight in an aircraft with sufficient range not to require at least 4-5 intermediary stops, I bet that aircraft will have more sophisticated means of navigation. But, hey, people do all sorts of crazy things Wink)

-- Chris.‌‌
By Stratajet - 1/8/2018 2:49:50 PM

ckurz7000 - 1/29/2017 8:21:40 PM
Dou you really rely on a VFR only tool like SD (with probably no map coverage for a transatlantic route) to perform such a flight??????? And if you really perform such a flight in an aircraft with sufficient range not to require at least 4-5 intermediary stops, I bet that aircraft will have more sophisticated means of navigation. But, hey, people do all sorts of crazy things Wink)

-- Chris.‌‌


I have used SD for two transatlantic VFR crossings and can confirm it works fine! The GC plot would have made it much better though since otherwise you have to use a different tool to work out what the track should look like and kind of make it up as you go! I also just did a trip to Norway where it would have been useful.
By rg - 1/10/2018 11:26:33 AM

I think the option to work either with GC or RL would be a nice addition to the software.  Particularly for planning and knowing that the lines will be in the same place as when the FPL gets entered into the Garmin. 

Personally I'd like to see 2 options. 
1 - use RL or GC (default to using RL).
2 - display both (default to off)

So if you select RL you have the option to also display the GC line in thinner or feint magenta.‌‌
By Tim Dawson - 1/10/2018 4:57:59 PM

So for every trip you'd like the option to specify whether to use GC or RL?
By rg - 1/10/2018 5:11:31 PM

Tim Dawson - 1/10/2018 4:57:59 PM
So for every trip you'd like the option to specify whether to use GC or RL?

Well software level setting I suppose.  You either prefer one or the other so doubt a use would toggle them often.  Certainly not per leg as was mentioned earlier in the thread.,
By T67M - 1/10/2018 11:03:04 PM

Definitely per SkyDemon instance, not per trip or per leg!

I‌'d make the solution even simpler: just have one option which is "show GC faint-line on/off". Routes would always be planned as RLs, and shown as such on the Pilot's Log, but if the show-GC option was on, underneath the thick magenta RL would be a faint magenta GC line. In many cases, the GC-line‌ would be completely invisible, but on longer east-west routes it would "appear" for those people who are interested and can be used to assist the insertion of additional waypoints if they would be helpful.
By Tim Dawson - 1/11/2018 12:19:46 PM

rg, the problem with that is loading a route planned by somebody else which has been specifically planned to be "safe" as a RL or GC when your software is setup with the opposite.

T‌67M‌ that idea is promising but for most cases where the GC differs not very much from the RL you'll get a weird small pink line only just protruding from the main thick line which will look odd. We'll get dozens of phone calls every week asking what on earth it is, if we turned it on by default.
By rg - 1/11/2018 12:25:44 PM

Fair point. So it’s route level then and leave everything as rl with gc hidden by default
By T67M - 1/11/2018 1:18:06 PM

I certainly don't think the show-GC option should be on by default, and if having a "bump" on the RL confuses people who have actively chosen to show GC to the point that they're calling your helpdesk then I'm disappointed and the level of PPL navigation being taught. 

To‌‌ help get around the helpdesk calls, maybe the GC line could be hidden completely using some simple algorithm, for example if the GC heading at either end of the leg differs by less than 1° from the RL, or if the mid-point cross track difference is less than 0.5nm? Couple that with a description of the line on the "what's here" popup and surely most pilots won't feel the need to call?
By ckurz7000 - 1/11/2018 2:05:23 PM

T67M - 1/11/2018 1:18:06 PM
I certainly don't think the show-GC option should be on by default, and if having a "bump" on the RL confuses people who have actively chosen to show GC to the point that they're calling your helpdesk then I'm disappointed and the level of PPL navigation being taught. 

To‌‌ help get around the helpdesk calls, maybe the GC line could be hidden completely using some simple algorithm, for example if the GC heading at either end of the leg differs by less than 1° from the RL, or if the mid-point cross track difference is less than 0.5nm? Couple that with a description of the line on the "what's here" popup and surely most pilots won't feel the need to call?


What's wrong with showing either one or the other? And if you are showing RL in a case where the course separation to GC is larger than, say, 0.5 nm, issue a warning in the "Warnings" tab.

-- Chiris.
By Sky Painter - 1/11/2018 3:29:13 PM

In the UK, at least, since the majority of routes are likely to be North/South, I think a GC option would be of limited value and I personally, don't see the need for it. However, I can understand only too well, the desire of others to have this option, so, how about:-

1‌.  Show one or the other – user selectable option in Settings, but default should be RL;

2‌.  When selected, show the RL route in magenta, as currently;

3‌.  When selected, show the GC route‌‌ in blue;

I‌ know that displaying a route line in a colour other than magenta it is a radical departure from the norm, but at least there could be no mistaking which is being displayed.

T‌im – In the case of someone loading a route created by another, would it be possible to check each user's setting and convert the downloaded route if necessary, or would this just be a coding nightmare?‌
By T67M - 1/11/2018 4:12:31 PM

The challenge I see with only showing a GC track (without the corresponding RL) is that it is impossible to create a GC-plog due to the varying heading that must be flown. That's why I favour keeping the RL as the core of both the SkyDemon chart and plog, but to additionally show the GC (in blue - I like that idea) on the chart to allow long RL sectors to be broken into shorter sectors which more closely approximate the shortest path. 
By Sky Painter - 1/12/2018 3:56:56 PM

T67M - 1/11/2018 4:12:31 PM
The challenge I see with only showing a GC track (without the corresponding RL) is that it is impossible to create a GC-plog due to the varying heading that must be flown. That's why I favour keeping the RL as the core of both the SkyDemon chart and plog, but to additionally show the GC (in blue - I like that idea) on the chart to allow long RL sectors to be broken into shorter sectors which more closely approximate the shortest path. 

You make a good and valid point.
By Tim Dawson - 1/15/2018 11:16:00 AM

It should be a solvable problem. Those who have been around these forums a while know that SkyDemon does support GC routes, and I released a beta once where all routes were GC just to test our code. It's finding the right UI to make the feature available but make no impact for the majority, which as SemperFi correctly says really don't know or care about the difference.
By Tim Dawson - 1/18/2018 10:35:44 AM

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.‌‌
By 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.
  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.
By 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. ‌‌
By T67M - 1/18/2018 1:18:10 PM

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.
By peter@luthaus.de - 1/18/2018 1:21:09 PM

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
By peter@luthaus.de - 1/18/2018 1:30:10 PM

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.
‌‌
By Tim Dawson - 1/19/2018 11:29:05 AM

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.
By Tim Dawson - 1/19/2018 11:30:43 AM

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.
By T67M - 1/20/2018 12:06:57 PM

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.
By ckurz7000 - 1/23/2018 9:09:32 AM

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.‌‌
By ckurz7000 - 1/23/2018 9:14:47 AM

peter@luthaus.de - 1/18/2018 1:30:10 PM

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.‌‌
By ckurz7000 - 1/25/2018 7:57:24 AM

Tim Dawson - 1/24/2018 3:03:45 PM
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.


Tim, you would still have that simplicity for 99% of the users as the difference between initial and final heading would in most cases be negligible. Furthermore, few people navigate by looking at the heading given in the PLOG and attempt to steer it. They simply follow the line on the map. And winds are always different than forecast, so headings will be different anyway.

Threrefore I believe that your concern isn't really a big issue at all. And since that seems to be the only thing holding back GC navigation, I don't see the big problem.

-- Chris.‌‌
By efrenken - 2/6/2018 4:01:02 AM

Just noticed, SD simulator navigates GC.

Flying a north-south route simulated aircraft follows exactly the magenta line (RL), west-east route it always deviates from the magenta line, hence my assumption.

Eric
By Stratajet - 5/16/2019 7:44:33 AM

So, it came... and then it went... 

I was delighted at the beginning of the week to read that GC functionality had been added to IoS testflight. With a mission planned for yesterday I decided to test it out.

By the time yesterday came however, it had been removed with a simple note saying "nobody seemed to care". Well, I definitely care!

I am a huge fan of SkyDemon. I love the way I can do flight planning on my PC and then have it on my iPad when I fly. I was chatting to a friend who also flies long distances and uses the leading competitor ipad software. When I asked why he didn't use SkyDemon is simply said "It doesn't do Great Circle". It's a known thing for those of us who fly any large distance.

I also remember flying up in Northern Greenland with SD and going way out of my way by following the track - before I realised I was following a RL.

If the functionality has been written to make it work then please can we turn it back on? Even if it's a hidden option somewhere it would be so helpful. It may make 1% difference to distance but when it's costing me £16 / minute to fly (PA31-350 - if you simply have to burn cash, accept no substitutes) over a year it will pay for many, many beers.


By rg - 5/16/2019 8:05:11 AM

I liked it on the route page as in the beta but would be just as happy as a user level setting in planning options.
By 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.
By rg - 5/16/2019 10:04:10 AM

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?
By Cat - 5/16/2019 10:30:24 AM

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.
By T67M - 5/16/2019 10:54:45 AM

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.
By Tim Dawson - 5/17/2019 1:59:11 PM

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.
By T67M - 5/17/2019 3:06:58 PM

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.
By ArnaudC - 5/29/2019 10:08:35 AM

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.
By peter@luthaus.de - 1/18/2018 10:02:41 AM

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.
By Tim Dawson - 1/24/2018 3:03:45 PM

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.
By jfw - 1/24/2018 3:51:34 PM

‌Just wondering, any reason why not implement option 1 of ckurz7000 proposal posted on 1/18/2018 12:00:02 PM ?
http://forums.skydemon.aero/FindPost25185.aspx
‌Would this not make everybody happy ?
By Cat - 3/22/2018 5:17:33 PM

Can I add one more vote for GC?

It would mean the track line and xte would match a paper plan.

As for the plog, take a leaf out of the likes of British Airways Cirrus plogs.

Initial true track and mean magnetic heading.

The heading will never vary by more than a few degrees. At 60°N, the Conversion Angle ½chlong sin lat for a 2° chlong, 60 Miles departure, roughly (chlong cos lat) is only .866° and none of us can fly that accurately. But the xte in the centre of that same leg is a more significant ½ mile.

Cat Burton
ATPL, ex BA, 30,000 hours and now FI and selling as many subs to Sky Demon as I can 😉
By Cat - 5/29/2019 10:31:22 AM

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.