Great Circles

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Great Circles

Author	Message
checklist	c checklist posted 7/19/2020 1:25:29 PM Post Details Share Post
c Group: Forum Members Posts: 7, Visits: 4	Just found on Skydemon facebook: And it works :-) Thank you. Great job !!! The next version of SkyDemon will use great circles when you plan a new route. Previous versions have always used rhumb lines. Unless you're planning journeys with exceptionally long legs (hundreds of miles), this will likely make no difference to you. For interest, though: for the same leg from A to B, a great circle will have a shorter distance and a rhumb line will have a steady track over the ground. If you're using a GPS to navigate with (as most now are), great circles make more sense. Routes saved from previous versions of SkyDemon will continue to use rhumb lines, so you'll see no change when you load a route you've carefully planned to avoid airspace and other features. Long great circle legs are identifiable by their curved appearance on a Mercator projected map.
	Reply Like 8
Tim Dawson	Tim Dawson posted 5/20/2020 9:42:44 AM Post Details Share Post
Group: Forum Members Posts: 8.1K, Visits: 9.2K	Definitely easiest on PC. Once you've made the change to your route though (on a PC) you can save it to the cloud and it'll be changed forever on any device.
	Reply Like 3
Hamish Mead	Hamish Mead posted 5/19/2020 3:25:33 PM Post Details Share Post
Group: Forum Members Posts: 19, Visits: 97	+x Tim Dawson - 1/24/2020 12:18:47 PM If you open any recently saved route file in Notepad you'll see CourseType="RhumbLine". Just change RhumbLine to GreatCircle and that sector will use great circles instead. Please go ahead and try this as much as you like... Happy to play/feedback, but please clarify: Do I export a route to a text editor, edit and re-import it to the SD tablet app, or is this feature only accessible via SD for PC?
	Reply Like 4
rg	rg posted 1/26/2020 5:44:47 PM Post Details Share Post
Group: Forum Members Posts: 109, Visits: 1.9K	Thanks for sharing that info. That's brilliant and looking forward to it just being the default method. Completely understand about legacy routes being stuck on rhumbline unless we apply this same workaround.
	Reply Like 5
Tim Dawson	Tim Dawson posted 1/24/2020 12:18:47 PM Post Details Share Post
Group: Forum Members Posts: 8.1K, Visits: 9.2K	The shipping version of SkyDemon actually has full support for great circle routes, there just isn't a way to switch to them through the user interface at the moment. A lot of testing on GC routes happened in the second half of last year. The rhumb line or great circle choice is per sector. If you open any recently saved route file in Notepad you'll see CourseType="RhumbLine". Just change RhumbLine to GreatCircle and that sector will use great circles instead. Please go ahead and try this as much as you like, and let us know if you think anything looks wrong. It's likely at some point in the future we may make it the default for new routes. Old routes must default to RhumbLine as that's how they will have been planned.
	Reply Like 10
rg	rg posted 1/24/2020 7:00:56 AM Post Details Share Post
Group: Forum Members Posts: 109, Visits: 1.9K	Just bumping this to enquire if we’re likely to see the feature back in a beta release anytime soon .
	Reply Like 5
Cat Burton	Cat Burton posted 5/29/2019 10:31:22 AM Post Details Share Post
Group: Forum Members Posts: 12, Visits: 116	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 SFI Aeros Cardiff Edited 5/29/2019 10:33:20 AM by Cat Burton
	Reply Like 8
ArnaudC	A ArnaudC posted 5/29/2019 10:08:35 AM Post Details Share Post
A Group: Forum Members Posts: 78, Visits: 137	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.
	Reply Like 4
T67M	T67M posted 5/17/2019 3:06:58 PM Post Details Share Post
Group: Forum Members Posts: 206, Visits: 3.2K	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.
	Reply Like 6
Tim Dawson	Tim Dawson posted 5/17/2019 1:59:11 PM Post Details Share Post
Group: Forum Members Posts: 8.1K, Visits: 9.2K	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.
	Reply Like 4