World projection used in ND map

[MetaMarty]

Hello all,

I have been wondering for a while what world projection model is used in the ND of modern aircraft like the 737 NG. I seem to remember that for regular tracks, the ND always draws straight magenta route lines. If the projection is Mercator, this would not be possible, since the line would need to be curved.

If it really draws straight lines only, which correctly follow the great circle track, I'm at a loss of what projection would be used.

So the question is, if you would enter 1 waypoint that is, say, 500 NM ahead, would you get a straight or a curved line in the ND?

Regards,

Marty

[Microburst2002]

I did wonder the same: what projection is used in NDs.

I would say it is like a polar stereographic, only its center is always the airplanes position instead of on the North pole. Straight lines would represent great circles, for practical purposes. Angles and distances between two points in the ND would very much resemble those of the real earth. At least any angle and distance from the airplane would be good. Meridians and parallels would appear slightly curved, I guess.

Anyone knows what do they use?

[End_of_Descent]

I don't have first hand knowledge.

That said, I've programmed a working ND for a simulation a few years back and I, too, found that a stereographic projection centered on plane position is the only practical solution that works everywhere with the same accuracy (in terms of distances, angles, as mentioned in the previous post), including N90 or S90.

EoD

[MetaMarty]

I'm also currently writing a complete 737 suite and without much thought implemented a Mercator view. For normal small legs this is fine, but when legs start to get larger than 80 NM, you will see the aircraft, which follows a perfect great circle route, start to move away from the magenta line and later catches up again. :{

I will look up the stereographic projection, but I'm still interested to know if the real aircraft draws arcs or straight lines only.

Marty

[STBYRUD]

The last 720NM direct I got (which started on a heading of 150 or so, starting point was somewhere in Sweden) was shown as a completely straight line on the ND, so I would vote for stereographic as well.

[aterpster]

Garmin G-1000 map at max range en route KLAX PHNL:

http://tinyurl.com/333l9h7

[Dan Winterland]

But that's in plan mode - and it's not an ND.

The one solution that makes sense to me is a transverse mercator which is based on the aircraft track. The reference track for the projection changes with the airacrft heading which is why the track stays straight.

This is what I have always supected, but I know the person to ask. I will post the reply when I have it.

[STBYRUD]

But that would mean that GC tracks displayed on it (such as the active route) would appear more and more curved as the aircraft turns away from it, I can't imagine that either. (Not that you would see this often though since one usually flies the magenta line...)

[Microburst2002]

If any of you is flying over the atlantic ocean you can make the following experiment. Select your EFIS to view the waypoints. Look at the waypoits of a given meridian, for instance 30º west. Can you put them all in the same straight line or not? (you can use a ruler or a paper)

[aterpster]

Dan Winterland:

But that's in plan mode - and it's not an ND.

Oh, but it is the ND in what Garmin calls the "North Up" mode. There are three other orientation options, Track Up, DTK Up, and HDG Up. Following is Track Up at long range scale; note the great circles still show:

I think you're stuck on the Boeing page. I was using Boeing's idea of a ND in 1984 on the early 767. A lot has changed since then, but not necessarily with birds Boeing certified eons ago.

http://tinyurl.com/2cwgfc6

[Old Smokey]

It's a Stereographic projection, with the point of projection the antipodes of the aircraft's present position.

Up to 720 nm it's quite good, but might show a few small (insignificant) kinks between waypoints. A Gnomonic projection would have have too much scale distortion and angular disparity at the greater ranges used.

Regards,

Old Smokey

[Dan Winterland]

I did ask the question of someone involved in the development of a navigation and display system. Obviously, he could only talk about his system - and it's quite an old one. He stressed that other systems were almost certainly different - but probably not radically so. And his answer is that we should not think too deeply about it! The navigation plan in the Flight Management Guidance Computer uses a spherical model and how they resolved to represent this on the ND wasn't the foremost of priorites at the design stage and wasn't considered too deeply. It's a visual guide only with no real thought to projection as they assumed no pilot would be getting his/her plotter out of their nav bag and placing it against the screen. The plotted waypoints, fixes, beacons and airfields are practically considered being a bearing and distance from the present position.

This system was designed to be used up to 320nm with later developements up to a range of 640nm. At 320nm, there wouldn't be a great deal of distortion. More so at the longer range. He mentioned that in practice, there would be few probelms associated with this as the 'to' waypoint would be on track and any subsequent waypoints would appear as such when they become the 'to' waypoint. I asked about a case where at the 640nm range, you wanted to fly direct to a distant waypoint off track using a HDG mode. He mentioned then yes, there would be distortion initially, but as the heading got close to the required track, then this would reduce and the pilot would naturally correct it. But it would be assumed that the pilot would engage a NAV mode anyway and the kit will be navigating on the spherical guidance computer model. He also mentioned that a pilot probably wouln't be aware of it as there were other factors such as drift to consider which would make any distortion virtually undetectable.

I then put it to him that once on track, the projection on the ND would be a transverse mercator with the origin being the airacrft track. He said that essentially that would be true, although he hadn't really though about it that way. He did stress that in this system, the aircraft was navigation on the most perfect map projection available - a globe - albeit an electronic one. The graphic display provided for the pilot was just a representation of that and should only be taken at face value as the aircraft will be navigating from the guidance computer and not from the display. He did also point out that when this system was developed, computers were considerably less powerful than they are now - and also that the displays of the time were all CRTs which had slightly curved screens anyway - which would not aid any notion of projection! Although there are more modern systems in use, this system is still installed in one of the todays best selling airliners and hasn't changed a great deal since introduction because of the costs of recertification.

So in answer to the original question regarding the 737NG, ''if you would enter 1 waypoint that is, say, 500 NM ahead, would you get a straight or a curved line in the ND?'' the answer according to my contact is straight. And having flown both Airbus and Boeing aircraft, I would tend to agree.

[Dan Winterland]

A quick Google found the two following quotes:

''clearly the principle of a pictorial navigation display is not tied to any one map projection'' (K.R. Honick, (1961) "Topographical Navigation Display: Description of the Experimental Instrument Developed and Constructed at the R.A.E., Farnborough")

And:

''Methods and apparatus are provided for an azimuthal equidistant projection (AEP) aircraft navigational display. The apparatus comprises a generator providing the aircraft's current latitude and longitude (φ1,λ0), a user input for providing the desired map radius R, a display for presenting the navigational information, a map data source providing topographical information associated with geo-spatial coordinates, and a controller and memory coupled to the generator, data source, input and display for providing data transformations. Map data containing topographical information associated with each geo-spatial coordinate is obtained from the map data source for an area of approximately radius R from (φ1,λ0). Using the controller and memory, the geo-spatial coordinates are converted to AEP spatial coordinates so that the topographical information is now associated with such AEP spatial coordinates. The transformed map data is sent to the display to provide AEP navigational information on the display around (φ1,λ0).'' (AZIMUTHAL EQUIDISTANT PROJECTION NAVIGATION DISPLAY AND METHOD United States Patent Application 20100017123)

Which seem to confirm it.