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Rhumb line navigation


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Timothy
Timothy
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If I do a relatively long East/West leg (yesterday was Biggin to Exeter, for example) if find that the line on SD is significantly displaced from that on the plumbed in GPSs (530/430/Pilot IIIc) to the extent that there is probably over a mile between them at the mid point. Thus, one can have a significant fly right on one and fly left on the other.

Is this because the GPSs use GC and SD rhumb?

I think that the standard is GC. This could cause an issue tracking an airway centreline, I guess.
Edited 10/15/2012 10:30:18 AM by Timothy
ckurz7000
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To decide whether SD is using a rhumb line or great circle route I plottet a single leg from 48N005W to 48N22E in SD. The course apears as a straight line, coinciding with the 48° latitude. This is a rhumb line. Therefore SD calculates rhumb lines rather than great circle courses.

If you want to track the centerline of an airway I suggest you pick intermediate waypoints.

I don't see this as a particular problem because it only ever results in significant deviations when you plot very long legs, running predomininantly in an easterly or westerly direction. Over a course from Brest (LFRB) to Nyireghaza (LHNY) the GC course is 1043 nm long as compared to the rhumb line course which comes out at 1047 nm. I can live with that. Just don't use it to cross the Atlantic.

-- Chris.
Timothy
Timothy
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It's only a problem in that it doesn't follow the standard, which is GC. Any GPS you use in tandem will use GC, meaning that the courses deviate.

Also, depending on how the line is calculated (start point, end point or midpoint) you can get a different track A->B than B->A.
Tim Dawson
Tim Dawson
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SkyDemon uses rhumb lines for planning and navigation. The track and heading shown in the PLOG is a rhumb line, because there isn't a decent way of representing a GC in the PLOG as by its nature, flying a GC requires periodic updating of the heading you're flying.

A long time ago we actually used to have a tool which helped you plot a series of rhumb lines tracking along a GC, but nobody used it so it eventually got removed when the parent feature was being updated.
ckurz7000
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Tim, I quite understand your reasoning. But you could show the course to steer at the current location. That course would update as you fly along, naturally. But at any point in time, when you view the PLOG, you'd see the appropriate course at this time.

-- Chris.
Tim Dawson
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We could change the DI/HSI instrument to work with great circles instead of rhumb lines, in theory. But what is shown on the map would still be rhumb lines, and the PLOG would also be giving rhumb line headings to steer. This sounds like a potentially confusing state of affairs to me.
Timothy
Timothy
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Tim Dawson (25/10/2012)
We could change the DI/HSI instrument to work with great circles instead of rhumb lines, in theory. But what is shown on the map would still be rhumb lines, and the PLOG would also be giving rhumb line headings to steer. This sounds like a potentially confusing state of affairs to me.
I agree that that would be the worst of all worlds.

It's not a big deal. Not many people come from my stable of putting in DCT however far they are going (I have been known to put in Biggin DCT Gothenburg, seeing no reason not to), and for a leg of anything less than 100nm it is barely noticeable, so I would just let it go.
Timothy
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Just to demonstrate that this is a non trivial issue, I took a screen shot of a track from Perranporth to Odiham, when tracking the GPS GC track:

http://ftp.artifax.net/misc/rhumbgc.jpg
Tim Dawson
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I can't really think of anything that would help you. What exactly is it that you're after here? We could have a tool that offered to turn a rhumb line into a great circle, putting points every 20 miles or something, I suppose.
Timothy
Timothy
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I am divided myself on this. I think that, on balance, SD should work the same as all modern GPSs and paint a GC. The track on the plog should be the initial track to follow the GC, and live plog used in flight should present the instantaneous heading.

I can't imagine that anyone who is using SD for planning only, and using DR in flight, would use long enough segments for the track to vary even a whole degree.

Edited to add that if the pilot uses radio nav to navigate, he will be flying GC, so again it will vary from the rhumb line in SD. (However, tracking a VOR becomes a special case because the actual track flown will be different from the nominated VOR bearing, by the convergence angle, but that doesn't matter right now.)
Edited 10/30/2012 6:25:49 PM by Timothy
2high2fastagain
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This is a really interesting problem. Timothy's map intrigued me and so I assumed I could buy a monster ferry tank and have a go at that Atlantic trip. The straight line in the pic below is the Skydemon 'direct' routing from East Midlands to Bangor Maine which looks like a rhumb line. The curved line is the GC route, crudely copied from Google Earth a few degrees latitude at a time. The GC distance is 2,617nm and the rhumb line is 2,707nm. Fascinatingly, the two lines are more than 5 degrees apart in the middle. Is that 300nm? But the distance flown by the rhumb line is only 3.04% longer. I think that to all practical purposes, the rhumb line is fine and if you ever attempted the Atlantic crossing in a single, it could be from airport to airport anyway.


http://i.imgur.com/dkD1a.png
Timothy
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if you ever attempted the Atlantic crossing in a single, it could be from airport to airport anyway.
Eh? Run that past me again?
http://www.smileyvault.com/albums/userpics/13617/scratch-head02-idea-animated-animation-smiley-emoticon-000415-large.gif

But actually distance flown isn't the issue.

If you flew across the Atlantic (and I seriously considered Iqaluit - Shannon, which I could just make, with my 600 litre ferry tank) you would definitely fly using the plumbed in GPSs. That means that if you had your SD on your lap, you would be, by your calculation, 300nm off track. Not a great back up if the electrics failed!
ckurz7000
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Excuse me but this discussion is largely theoretical.

1) In 99.9% the difference between rhumb line and GC track is not an issue.

2) In those cases where it might be you can always put intermediate points on the GC track and approximate the GC course by several rhumb line legs.

So we are talking about the 0.1% cases for which the work-around solution is impractical.

-- Chris.
Timothy
Timothy
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I agree it is theoretical.

It's just a question of industry standard, how aeroplanes fly, how radio aids propagate and how all aviation GPS' are designed.

For example, SD could never be RNP certified, for exactly the reason 2high2fastagain gives.

There are more important things for Tim to focus on, such as getting all the airway centrelines in the system, even 3D SIDs and STARs, but that doesn't mean that it shouldn't be mentioned, just to sit on the back burner and simmer.
2high2fastagain
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Timothy - LOL - I'm sure you know what I mean! Try 'short hops from airport to airport anyway' instead.

In terms of that big crossing, I'd concede it would cost a few bob to hire the two Queen Elizabeth aircraft carriers (when they are finished) and to strategically position them as refueling stations for that weekend trip to Bangor Maine (and we'd be the only aircraft on them LOL)

Yes, this is a theoretical thread and I completely agree there are more important things for Tim D to do, but nonetheless it's still interesting.

The thread drew my attention as I would like to attempt the Atlantic Crossing one day (perhaps to fly into Oshkosh). Timothy, I'd go via Iqaluit as well with a thumping great ferry tank in the C182, though my routing would be Wick, Reykjavik (BIRK), Kulusuk, SondreStrom, Iqaluit and then down through Canada. Having worked in Greenland and seen the spectacularly rapid weather changes, my criteria would be that I would like to have enough fuel to go back to where I came from or even further back if the fog rolled in (I'm a project manager not an adventurer). But that discussion is one for another forum.

In terms of constructive advice for Tim D. Perhaps if someone puts in a very very very long track, a gentle warning that you are getting a rhumb line would be an ample response from Skydemon, but I wouldn't stick it at the top of any lists.
Timothy
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2high2fastagain (07/11/2012)
I'm sure you know what I mean! Try 'short hops from airport to airport anyway' instead.
I think you are missing my point. How many airports are there on either of the tracks you have drawn?

My longest single flight not passing a single airport was 1385nm, though it has to be said that the GC coincided with the rhumb on that one Laugh
2high2fastagain
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None of course, which is why I at least, would take short hops across the northern route in a single.


Richard747
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I would like an option for SD to display the GC, the Plog to show Initial True/Mag Track and a changing en-route track required to follow the GC. Thanks.
Tim Dawson
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I've been toying with the idea of showing the GC from takeoff to landing if (for example) its length deviates from the rhumb line length by a certain amount. Would that be useful?
Richard747
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Yes, that would be a step in the right direction - thanks.
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Timothy - 10/15/2012 10:29:59 AM
ckurz7000 - 10/16/2012 1:00:11 PM
                         It's only a problem in that it doesn't follow the standard, which is...
Timothy - 10/16/2012 1:20:28 PM
Tim Dawson - 10/18/2012 1:00:43 PM
                         Tim, I quite understand your reasoning. But you could show the course...
ckurz7000 - 10/18/2012 5:31:47 PM
Tim Dawson - 10/25/2012 3:53:26 PM
                         [quote][b]Tim Dawson (25/10/2012)[/b][hr]We could change the DI/HSI...
Timothy - 10/26/2012 9:15:09 AM
Timothy - 10/28/2012 12:33:43 AM
Tim Dawson - 10/30/2012 10:30:19 AM
Timothy - 10/30/2012 6:18:02 PM
                         This is a really interesting problem. Timothy's map intrigued me and...
2high2fastagain - 11/6/2012 8:42:50 PM
                             [quote]if you ever attempted the Atlantic crossing in a single, it...
Timothy - 11/6/2012 11:03:28 PM
                                 Excuse me but this discussion is largely theoretical. 1) In 99.9% the...
ckurz7000 - 11/7/2012 9:04:54 AM
                                     I agree it is theoretical. It's just a question of industry...
Timothy - 11/7/2012 11:08:14 AM
                                         Timothy - LOL - I'm sure you know what I mean! Try 'short hops from...
2high2fastagain - 11/7/2012 11:46:14 AM
                                             [quote][b]2high2fastagain (07/11/2012)[/b][hr]I'm sure you know what I...
Timothy - 11/7/2012 5:31:40 PM
                                                 None of course, which is why I at least, would take short hops across...
2high2fastagain - 11/8/2012 8:44:41 AM
                                                     I would like an option for SD to display the GC, the Plog to show...
Richard747 - 12/12/2013 5:52:21 PM
Tim Dawson - 12/13/2013 12:14:26 PM
                         Yes, that would be a step in the right direction - thanks.
Richard747 - 2/14/2014 10:43:53 PM

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