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On Tue, 05 Dec 2006 01:10:27 -0700, Bill  wrote:

> 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.
>>
Correct.  I had neglected to read the whole article before posting the
location, and the author did "tests" and posted results (as described
later) to determine that the algorithm cut off in 1995.


> 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.
>
I think the old Garmin algorithm was to the nearest degree . . . and given
the various magnetic anomalies (up to 90 degrees!!) and local attraction,
you would be better off with a good map, a sextant, and a good variable
angle protractor to determine accurate bearings or angles between
objects.  Although I do like my various bearing compasses which read to
the degree or half degree (and can be estimated somewhat more finely than
that) I find even the screws in my eyeglasses, the pens and pencils in my
shirt pockets (and everything else that I have forgotten to remove) will
distort the magnetic field around me.  The difference in the angle
differences as taken from a theodolite (between distant landmarks)
compared to those derived from subtracting magnetic azimuths from the
bearing compasses are great enough to be cautious about accepting even
that half degree reading.

The sextant is much better, but since I did my testing a long time ago
with a plastic Davis vernier example, the results may not be meaningful.
Besides, the theodolite measures angles using a vertical hair through each
object, and the sextant had to be tipped upward to superpose the objects
while attempting to maintain their relative elevation.  (I'm probably
confusing the issue here by going off on side tracks not really applicable
to Navigation.)

--
Richard . . .
Using Opera's e-mail client since Dialog, "the Dog", died.

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