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This question arises from the Garmin GPS units (eg. GNS530 or GTN750), but the term ground track appears in a general context of GPS units so my question is also general.

Is the ground track (TRK in Garmin) a magnetic or a true course or does it depend on a specific GPS unit?

Intuitively, ground track should be a true course as it is (presumably) computed from GPS coordinates over time. But I can't find an authoritative (or any) source discussing how the ground track is computed in GPS units and is it true or magnetic. I'm specifically interested in Garmin units, but any information will be useful.

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    $\begingroup$ "Intuitively, ground track should be a true course as it is (presumably) computed from GPS coordinates over time. " -- just keep in mind that these units no doubt have a stored world map of magnetic variation, so it would be a simple matter to "correct" from True to Magnetic or vice versa. $\endgroup$ Nov 25, 2023 at 14:27

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The Garmin units display their ground track as magnetic heading.

Source: We recently performed a test-flight with in which we compared the Garmin display to our experimental test recording devices (including GPS and INS) because our test pilots pointed out that some discrepancies between the Garmin "Track" and the true course heading which we calculated. However, I should point out that we could not find an according mention in the handbook of the Garmin device.

That Garmin chose magnetic Heading as the "Track" angle makes a bit more sense if you think about how pilots fly: If they speak about heading they pretty much always mean magnetic heading, because that is what their instruments display. If Garmin would choose to display the true heading, Pilots would be very confused, why their heading deviates by a couple of degrees from what their instruments would display...

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  • $\begingroup$ The GPS is probably calculating a great circle route, which is generally NOT a constant true (nor magnetic) course... the no-wind heading needed to maintain that GC route will change along the route. So if you've computed "the" TC between 2 points, it's to be expected that the GPS won't always agree, especially over long distances. $\endgroup$
    – Ralph J
    Nov 25, 2023 at 6:23
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    $\begingroup$ @mins Yes, not the entire great circle, but the salient point that its heading changes along the track remains. $\endgroup$
    – Ralph J
    Nov 25, 2023 at 16:15
  • $\begingroup$ I'm not a pilot other than taking ground school many years ago, but I thought ground track and course are tied together, i.e. they are referring to the actual travel over the ground based on true north. Meanwhile heading is which way the nose is pointed and normally based on magnetic. For example if flying between two airports that are at the same longitude the ground track would be north, although due to crosswind the heading might be slightly northeast. I'm wondering why a pilot would be confused if the GPS showed their ground track as 360 instead of a magnetic number. $\endgroup$ Nov 25, 2023 at 17:07
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    $\begingroup$ For aviation applications, for reasons given in the your answer, in most cases using "magnetic" headings makes the most sense-- yet in some applications-- specifically navigation very near the magnetic poles -- it seems that "True" headings might be much more useful. I'd be surprised if dedicated aviation units gave the pilot no option to switch to "True", but can't speak from experience. – $\endgroup$ Nov 27, 2023 at 0:16
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Is the ground track (TRK in Garmin) a magnetic or a true course or does it depend on a specific GPS unit?

In some Garmin non-aviation-specific handheld GPS units (e.g. 76S, and most of the Etrex series) the function you are speaking of is called "Heading" rather than "TRK". Since these units lack magnetic sensors, it is (despite the name) computed entirely based on the very recent (i.e. near-instantaneous) ground track. The user may select whether the displayed value is a Magnetic value or True value.

In related variants that also include magnetic sensors (e.g. Etrex Legend), the user may switch off the magnetic sensor entirely, or may select a minimum groundspeed value below which the "Heading" value will be based on on the magnetic sensor rather than the ground track.1 If the magnetic sensor has not been switched off, above the selected groundspeed value the "Heading" value will be based on the ground track. The logic of this design is clearly that with a handheld unit, optimized for use while hiking etc and not intended to be used to infer wind (or current) correction angles, above a certain groundspeed, track-based information is more reliable than magnetic-based information.2 In such cases, the user's selection of Magnetic or True information for the "Heading" value will apply regardless of which of the two modes (ground track-based or magnetic sensor-based) the unit is operating in.

My experience has shown that the units mentioned here, while not designed specifically for aviation, can actually be extremely useful in various non-IFR aviation applications, including cross-country navigation over a cloud deck, and soaring. All the units mentioned here have the ability to display current glide ratio and glide ratio to destination, which is very handy for soaring. Wind direction can be observed (and wind speed can be estimated) by the direction of drift shown in the plotted ground track while thermalling (e.g. while flying in multiple circles), or alternatively by taking a moment to find the "Heading" that yields the lowest groundspeed. A nice feature of the 76S in particular is its ability to display up to 12 numerical data fields on the same page as the map display. For soaring in particular, it is often convenient to mount the unit on a small board that straps to the pilot's leg. Just be certain that if the particular unit does have a magnetic sensor, it is switched off-- for the reasons given in footnote 2, a simple 1-axis magnetic sensor is extremely impractical to use in flight. The aircraft's wet magnetic compass, or gyroscopic heading indicator (if present), will serve much better if the pilot wishes to compare the aircraft's ground track with the aircraft's actual heading. The magnetic heading sensing function on dedicated aviation GPS units is obviously based on more sophisticated sensors and algorithms than are present in these simple hand-held units.

Footnotes:

  1. "Groundspeed" is my term; these units just use the term "Speed".

  2. Note that unless held perfectly level, the simple 1-axis magnetic sensor in these units suffers from "tilt" errors that are very similar to those experienced by a conventional "wet" aviation compass. In actual practice these make the magnetic-sensor-based "Heading" function even more difficult to interpret than a conventional "wet" compass during banked, turning flight.

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