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 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. Since supporting this might be much cheaper than I originally thought, we just need to come up with a way of the user selecting which they would prefer.

www.skydemon.aero/start/beta.aspx

Ok, I just wanted to make sure we were on the same page. The cross track error is only an issue if you are steering great circles. SkyDemon helps you plan, and steer, rhumb lines. At present we have no plans to help you plan and steer great circles. Helping the user to form a route comprised of consecutive rhumb lines that follow a larger great circle is something I'm open to however.

While I agree that for all practical purposes, the difference between GC and RL is irrelevant for most users of SkyDemon, I think the original query was more concerned about the difference in track over the ground than the difference in track length. What does the result look like if the threshold is set to a cross-track difference of more than 1nm (25% of a small ATZ) at the mid-point of the leg? (And I wish I was clever enough to work this out myself!)

So this morning I set out with intentions of prototyping a feature where the GC line between your takeoff and landing airfields is drawn faintly in the background, if the GC is more than 5% more efficient than the rhumb line. It didn't work, because although I created routes spanning the whole of Europe, I couldn't find any where there was that much difference. In fact I barely managed a 1% difference.

The map projection doesn't have anything to do with whether legs are rhumb lines or great circles internally.

Had you considered changing the map projection at smaller scales, or is that just too hard?

I will add that since the vast majority of SkyDemon subscribers appear to be happy with the current method, we would need a mightily compelling reason to switch at this stage. The current method gives the pilot a heading that, if he/she flies it, will get him/her from A to B (subject to wind). That simplicity is worth a lot, and that is what the notion of switching to great circles per leg is competing with. If you're navigating using SkyDemon as your GPS (which a huge percentage of our customers are) it doesn't matter, because the navigation instruments will also be helping you along the rhumb line.

Internally, SkyDemon uses both rhumb lines and great circles all over the place. In a very early beta, each leg in a route could be comprised of either. We had to ditch one of them because the choice was too confusing. We ditched great circles because a fundamental requirement of SkyDemon has always been to produce a PLOG with a heading to fly for a leg. You can't do that with great circles.

I'm open to re-introducing great circles into the mix, but not on a per-leg basis. I can imagine, for example, that if somebody planned an exceptionally long leg like the one shown above, where flying a great circle would give an appreciable fuel saving or where the midpoint different from the rhumb line midpoint to the great circle midpoint exceeded a given distance, we would ALSO draw the great circle on the map. This would give a visual clue that the leg could be "broken up" by the insertion of more turning points to create more rhumb lines. Or we could draw the great circle between takeoff and landing airfields for instance.

Hey there,
i’ve been following this discussion quite a while. I can’t see the point in not willing to program it the right way. Every flight we do is a great circle. On small distances you might not see a difference. Flying from here to a place 100NM or further you’ll notice a difference. I cant get your point of view, saying it is a VFR only tool. How does it make a difference depending on the rules you fly? The problem is still the same isn’t it?
Talking to other pilots and friends, especially those who fly all over Europe, would appreciate if Skydemon will give an update, cause it is not a feature, it is a bug! Looking at Jeppesen and alle the other tools you’ll notice, that they all use great circles, cause thats what you fly out there. I really do get Peter's point of view and share it.
Please fix this bug, i really appreciate.
Safe flying aviators,
Lars

For the PLOG I would not care to leave it as it is, the mean course and heading would be fine. 

You can't have it both ways! The "mean course and heading" of a great circle route is the rhumb line to all intents and purposes. Are you now arguing that the PLog and the magenta line should be different? I see airspace busts ahead using that approach!

There are two scenarios here:

1. The route leg being flown is short enough that there is no significant difference between the two methods. By definition the same heading will be used for both rhumb line and great circle navigation. This represents a significant portion of legs flown by people using SkyDemon.
2. The route leg is long enough that the differences are noticeable. By definition, the great circle route must have different initial and final headings (if it didn't, it would be a rhumb line!)

Tens of thousands of pilots (i.e. the current users of SkyDemon) seem to be happy to use the single heading rhumb line offered by SkyDemon and as taught during PPL navigation training. Most of them won't even notice the difference, and those who notice mostly don't care provided that the magenta line shown on their SkyDemon device represents the heading shown on their PLog. I suspect that the majority of these pilots would be confused if SkyDemon suddenly offered them two (or more) headings for each leg. 

There are a small number of pilots would prefer the mathematical perfection and/or want to benefit from the fuel/time savings (typically less than 5%) of a great circle route. In return they accept complexity and higher workload of flying a constantly changing heading. I have no objection to them choosing to do so if they wish, but I would strongly object if this small number of pilots caused confusion to the tens of thousands of people who are happy with the simpler (albeit mathematically imperfect) solution of flying a rhumb line as they were taught during their PPL training. That is why great circle navigation has to be a configuration option - if it is ever supported at all.

At the end of the day, SkyDemon is Tim's product, and he will implement the features he feels satisfy the needs of the majority of his customers. Thus far he has been incredibly successful - thanks Tim!