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On 11/15/2012 12:23 PM, Gary LaPook wrote:
> You don't want any ferric metal near the compass when you are
> determining the deviation. If you wear a watch make sure put that arm
> behind you back and if you wear glasses with metal in the frames take
> them off. Larger pieces of metal (such as your rifle) need to be at leat
> 10 meters away and power lines at leat 150 meters. If you are near a
> street, and you are using a quality compass like an M2 aiming circle or
> a transit, you will see the compass needle move when a car drives by.

I definitely see magnetic influences from water lines, my Hobie trailer
etc. around my home.

I once had the compass on a houseboat way off due to an aluminum
beverage can close to the compass.  I did not see that coming!

I often test for south or north by using the meridian passage of a body
and the declination/variation calculated from the site Gary mentioned. I
also update my charts in areas where I might be cruising (Lake Michigan)
at the beginning of each season.

My bushwhacking compass allows me to dial in declination for a given
region, which is handy.

I have never had any success getting a good instantaneous (true) bearing
from a GPS on a sailboat as the motions throw the GPS off. I do not
trust the GPS for magnetic bearings as I have been unable to get a clear
answer from Garmin on how magnetic is calculated from true.  If it is a
lookup table added to the firmware when the unit was manufactured or
firmware updated, it will be out of date quickly. Of course one has the
option of setting the variation manually, but that is a pain given the
change from the east to west coasts of Lake Michigan.  More so up the
right or left coasts of North America.

Bill B

   

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