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I hope this isn't a dumb question, but I haven't flown a whole lot of GPS approaches and today I observed something that I cannot explain: There was a 2 degree split between the Desired Track (DTK) displayed on the GPS, and the inbound course shown on the approach plate.

If it matters, the approach was the RNAV 16 Y approach at KRNT, and I was using my club airplane's Garmin GTN 650. The database in the GPS was up to date, and my approach plate in Foreflight was current. DTK showed 159, and the plate showed 157.

Not a big difference, but they ought to be the same, right?

At first I thought it might be displaying track direct from my present position, but I had early turned the fix a little coming from the west, and was lined up a bit right which would have made the course needed to correct a few degrees less than 157.

And as I made minor course corrections back and forth across the final approach course the DTK never wavered.

Is there a good explanation for this difference that I'm just not aware of?

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    $\begingroup$ How old is the approach (i.e. when was the chart last updated)? Perhaps the local variation has changed enough over time that the mag course to get from the FAF point to the runway is now, rounded to the nearest whole degree, 2 degrees different? From, say, 157.4 degrees (rounded to 157) to 158.6 (rounds to 159). $\endgroup$
    – Ralph J ♦
    Jul 17, 2020 at 3:26
  • $\begingroup$ @Ralph J, the plate says Aug 17. I guess that would explain it, but isn't that why they update things? $\endgroup$
    – Michael Hall
    Jul 17, 2020 at 4:07

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Here is the final answer handed down directly from the Garmin technical support department. After a conversation with one of their aviation technicians, he told me that the error seems to be in their database. The same discrepancy (a track of 159 instead of 157) appeared whenever he reviewed the KRNT 16 RNAV Y approach from his end. He even rolled back the database to check previous versions and it has been this way for quite awhile. When asked about a possible magdev discrepancy, he said they they acquire all of their deviation data from the same governmental source that the chart makers do. He said that they are releasing a magdev update in the near future that MIGHT fix the problem, but said that it was most likely just entered wrong in their database. He opened a case file to make sure it is addressed and said that he will notify when it has been taken care of.

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    $\begingroup$ You have gone over and above, thank you!!! $\endgroup$
    – Michael Hall
    Jul 18, 2020 at 20:44
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    $\begingroup$ Of course! I was very curious to know the answer myself. $\endgroup$
    – Aaron Holmes
    Jul 18, 2020 at 20:46
  • $\begingroup$ I don't think this is the answer. Two years later there is still a 1-2 degree difference between most RNAV approach charts and Garmin's database. See my answer, from the AIM. $\endgroup$
    – CpnCrunch
    Feb 25 at 15:23
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@Ralph J was on the right track with the hidden decimals behind the degrees. This should solve the mystery:

You’ve probably noticed that GPS course guidance is often a degree or three different from the courses shown on the chart. The reason is the magnetic variation used by your GPS, the chart designer, and the VOR defining an airway may all be different.

Approach designers calculate to the fraction of a degree. This means a transition route might start at 141.4 and end at 141.2. The whole route appears as 141 on the chart, but those tenths of a degree measured at the actual turning fix might be the difference between a legal 89.9-degree turn, and a forbidden 90.1-degree turn.

The article is entitled A Bit Over Ninety written by Jeff Van West at IFR Magazine.

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  • $\begingroup$ I think we are getting close, but it has nothing to do with VOR airways. This is an apples to apples comparison - GPS to GPS. Two waypoints, roughly 10 miles apart, with a 2 degree split depending on what you reference. The only real explanation is that the GPS nav system is drawing from a different dataset than the chart creator. But shouldn’t we expect a level of standardization to prevent this? $\endgroup$
    – Michael Hall
    Jul 17, 2020 at 22:43
  • $\begingroup$ Please check my math, but isn’t that around 2095 feet?! $\endgroup$
    – Michael Hall
    Jul 17, 2020 at 22:50
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    $\begingroup$ But it doesn't have to involve a VOR airway. That's just one possible source of variation. The magnetic variation between your GPS and the chart designer could still be off. $\endgroup$
    – Aaron Holmes
    Jul 18, 2020 at 1:27
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    $\begingroup$ However... that’s not the issue here. The problem is that you are thinking like a pragmatic human, not like a Garmin 650. If you loaded the RNAV approach, your GPS doesn’t aim directly from LUTSY to TOYOL and use the intermediate fixes for vertical descent reference. It vectors you from LUTSY to BLEIR, then BLEIR to ZULAV, then ZULAV to TOYOL. When you hit LUTSY and it said 159, it was calculating DTK to BLEIR, which was 5.9nm away, not 10.3nm. Thus, the margin of error was smaller, approx. 790 ft instead of 2,100 ft. $\endgroup$
    – Aaron Holmes
    Jul 18, 2020 at 4:20
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    $\begingroup$ It has nothing to do with the curvature of the earth. It has to do with distance between points on radials that are equidistant from a set waypoint. Think of it like this: You are on the 270 radial of a point that is due east and 10 miles away. To find your distance from a point on the 272 that is ALSO 10 miles away, you calculate the base of an isosceles that is opposite the 2 degree angle. However, if you are flying due north and want to know where you will intercept 272, you calculate it as a right triangle. $\endgroup$
    – Aaron Holmes
    Jul 18, 2020 at 12:45
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This is explained in the FAA AIM 1-1-6.b.5(j):

Impact of Magnetic Variation on PBN Systems

(1) Differences may exist between PBN systems and the charted magnetic courses on ground-based NAVAID instrument flight procedures (IFP), enroute charts, approach charts, and Standard Instrument Departure/Standard Terminal Arrival (SID/STAR) charts. These differences are due to the magnetic variance used to calculate the magnetic course. Every leg of an instrument procedure is first computed along a desired ground track with reference to true north. A magnetic variation correction is then applied to the true course in order to calculate a magnetic course for publication. The type of procedure will determine what magnetic variation value is added to the true course. A ground-based NAVAID IFP applies the facility magnetic variation of record to the true course to get the charted magnetic course. Magnetic courses on PBN procedures are calculated two different ways. SID/STAR procedures use the airport magnetic variation of record, while IFR enroute charts use magnetic reference bearing. PBN systems make a correction to true north by adding a magnetic variation calculated with an algorithm based on aircraft position, or by adding the magnetic variation coded in their navigational database. This may result in the PBN system and the procedure designer using a different magnetic variation, which causes the magnetic course displayed by the PBN system and the magnetic course charted on the IFP plate to be different. It is important to understand, however, that PBN systems, (with the exception of VOR/DME RNAV equipment) navigate by reference to true north and display magnetic course only for pilot reference. As such, a properly functioning PBN system, containing a current and accurate navigational database, should fly the correct ground track for any loaded instrument procedure, despite differences in displayed magnetic course that may be attributed to magnetic variation application. Should significant differences between the approach chart and the PBN system avionics' application of the navigation database arise, the published approach chart, supplemented by NOTAMs, holds precedence.

(2) The course into a waypoint may not always be 180 degrees different from the course leaving the previous waypoint, due to the PBN system avionics' computation of geodesic paths, distance between waypoints, and differences in magnetic variation application. Variations in distances may also occur since PBN system distance-to-waypoint values are ATDs computed to the next waypoint and the DME values published on underlying procedures are slant-range distances measured to the station. This difference increases with aircraft altitude and proximity to the NAVAID.

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  • $\begingroup$ With all due respect, this is not the answer. The difference in this case is NOT between a PBN system and a course to a ground based navaid. It was an observed difference between the desired GPS track of an approach loaded from a current database of published approach procedures, and that same track depicted on the plate of the published procedure. $\endgroup$
    – Michael Hall
    Feb 24 at 6:14
  • $\begingroup$ The AIM entry is not just discussing ground based navaids: it is also talking about SIDs, STARs and approach charts (see the first sentence), and those may or may not be based on ground based navigation aids. The key point is that the chart designer and the database might use slightly different variation: "PBN systems make a correction to true north by adding a magnetic variation calculated with an algorithm based on aircraft position, or by adding the magnetic variation coded in their navigational database." $\endgroup$
    – CpnCrunch
    Feb 25 at 15:30
  • $\begingroup$ But why would a chart designer use a MagVar different than the database? (Or vice versa) That’s the root of my question, and according to Aaron Holmes’ conversation with Garmin it appears to be an error. $\endgroup$
    – Michael Hall
    Feb 25 at 18:37
  • $\begingroup$ That AIM entry suggests it is because there are different ways of calculating the variation, and perhaps Garmin is more accurate. You can see most charted rnav approaches are a few degrees off from the chart today, so it's clearly not a one off error. $\endgroup$
    – CpnCrunch
    Feb 26 at 20:32

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