Dave Butler wrote:

wrote:

Ron Natalie wrote:


wrote in message ...

I often see 2 or 3 degrees difference between the Garmin 530's desired track
along an IAP segment, and that on the approach chart. Of course, it doesn't
matter since the IAP string is a flight plan between waypoints.

That's to be expected. Even if there were no magnetic variation, the track
(except in certain degenerate cases) must change as you fly along any straight
line.



Not for a 5 or 10 mile IAP segment.


I know the FAA folks who publish the IAPs consider the magnetic courses they
show on the official IAP document to be "guidelines," rather than the precise
magnetic bearing between IAP waypoints.

Nope, they're pretty danged accurate. You just have to realize that the bearing
relative to a station must change while tracking a straight line.


Then, why are the chart and the Garmin often in disagreement for a 5 to 10 mile IAP
segment?

Well, it might be that they are using different values for the local magnetic
declination. For example, suppose NACO uses a measured declination from an FAA
check flight, and Garmin uses a calculated value from a best-fit polynomial.



Exactly, as I said earlier in the thread.