How do you determine the correction for a magnetic compass?

 

Specific Question:

 

I bought a watch that has an electronic compass and in the manual it says:
 
Declination adjusts the compass for the local magnetic fields. In some areas,
magnetic declination can be considerable. Declination is sometimes measured as
degrees negative and positive. Degrees East are considered positive and degrees
West are considered negative.
 
Answer:
 

Subject:

RE: RE: declination angle

Date:

Wed, 18 May 2005 17:12:21 -0400

From:

"Canali, Eric"

To:

gawalsh@planetarium.cc

Top of Form

Bottom of Form

 

I believe that the "declination" used in this case is the term used for

the angle at which the Earth's magnetic field is skewed in your local

area relative to true North. The magnetic pole is not located at the

North Pole but rather is down in the upper reaches of the Hudson Bay area.

The lines of magnetic force radiate outward from that general

area and are by no means smooth and regular. The local value used to be

marked in the margin of USGS Topo maps (topographic maps issued by the

U.S. Geological Survey; such maps may be available at local public or academic libraries),

and it varies wildly in certain areas of the country. As I remember, there are some areas

less than 100 miles from here (Pittsburgh) where a magnetic compass can be OVER 90 degrees

wrong and will point SW (southwest) instead of true north for example [my source:

a map of USA magnetic declination in the 1958 Official Boy Scout Manual (!)].

 

Magnetic compasses are notoriously useless in the hands of anyone

without some significant training -- i.e.: geologists, retired surveyors, and

old (Boy Scout) merit badge candidates.

 

- ERIC CANALI

 
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Editorís Note: This Science question was answered by a very experienced amateur astronomer,
who also works as a part-time Tour Guide at the Allegheny Observatory in Pittsburgh.
Allegheny Observatory provides public tours two nights a week from April through October;
pre-registration is necessary, but the tours are free-of-charge.
 
For many years, Eric G. Canali was Floor Manager for the original Buhl Planetarium
and Institute of Popular Science in Pittsburgh. He also was a long-time member of the
Amateur Astronomers' Association of Pittsburgh and is Founder of the South Hills
Backyard Astronomers.
 
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gaw