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Richard wrote:

> Most GPS receivers have internal data and an algorithm to compute the
> declination after the position is established. For example, the Garmin
> GPS-45 displays the value on the Navigation Setup page, Heading: "Auto
> Mag," and uses it to supply magnetic bearings. However; this data cannot
> be updated from satellite transmission, therefore it is subject to become
> outdated.

If I recall--a long shot--the 45 is very old technology.  The predicted
movement of magnetic north on some of my older charts has not lived up to
expectation according to current US Gov sites. Therefore any dated
algorithms would suffer the same fate as the chart mag vs. true predictions.
>
> When asked about this issue for the 12XL, Garmin responded:
>
> "The magnetic variation model used within our products is a series of
> tables derived from the NOAA IGRF '90 model, which accounts for the
> movement of the magnetic poles over time."

If I understand, an old algorithm again.  I have noted on the true to mag
options on my Garmin 12 I have a choice of auto or user defined variation.
I will have to try plugging in current up-to-date user defined values from
the US gov site.  I doubt given that given the  resolution (1 d) and
previous comparisons that as few seconds or a minute will get it spot on,
but an interesting exercise.

Thanks

Bill


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