On the earth's surface, a compass points to the magnetic North Pole rather than the geographic North Pole. The angular difference between these two directions is called magnetic declination.
Magnetic declination varies across the earth's surface and over time. Presently, in Wisconsin we experience very small values for magnetic declination,
but this has not always been the case. The information that follows is a condensed version of a 4-page guide called Wisconsin Magnetic Declination, that was produced by the SCO in 1999. The full version of this guide contains a mixture of text and graphics including several maps illustrating magnetic patterns over the state. This and other guides are available free upon request from the SCO and online as PDF documents.
• Magnetic Declination
• Using Magnetic Declination
• Magnetic Deviation
• Magnetic Variation
• Calculating Magnetic Declination
• Related Links
Magnetic Declination The angular difference between the geographic and the magnetic meridians at a point on the earth is called magnetic declination. The values for
magnetic declination in Wisconsin vary today from -4.0° in Door County to +1.5° in the west-central part of the state. While these values are
not as great as the +20° declination in Seattle, Washington, they are still of concern to surveyors, map makers and navigators, and others who
use maps or compasses.
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Using Magnetic Declination
To translate the difference between the geographic and magnetic meridian, the angle of declination must be subtracted from the magnetic north reading. For example,
Eau Claire, located west of the 0° line, has a magnetic declination of approximately 1° 30' east (as of 1998). To correct a magnetic compass reading to true north when west of the 0° line, the declination
value should be subtracted from the magnetic north reading. When east of the 0° line, you should also subtract the declination from the magnetic north reading, but remember that in this case the declination value is negative. Subtracting a negative number will essentially result in adding the declination value to the magnetic north reading.
Where the magnetic declination is zero, a compass needle will point true north.
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Magnetic Deviation Return to Top
Magnetic Variation Return to Top
Calculating Magnetic Declination The NGDC also offers an online magnetic declination calculator program called Historical Declinations of the United States. This program is designed only for the conterminous U.S., but allows for a greater time span beginning in 1750 and running to 1995. This program only returns horizontal declination. Also, since this program only calculates in the western hemisphere, you enter the longitude as positive.
If you prefer to have a simpler graphical reference, the U.S. Geological Survey produces a map of the U.S. showing lines of equal magnetic declination
and annual change. Additional points of magnetic anomaly are also posted on this map. The map is revised every five years or so to compensate for the changes in the magnetic field. The most recent map is from 1995 though only downloadable page-sized versions are available. It is unclear whether the USGS plans on continuing this map.
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Related Links and Resources
Finding your way with map and compass, provided by the USGS, is a short and informative guide to using a compass in the field.
Many books on the subject of orienteering or navigating with a compass can be ordered on the Internet. Your local bookstore or camping outfitter is also likely
to carry books on the subject.
For further information about magnetic declination and other mapping-related topics, contact the SCO.
The line of 0° declination currently passes through Wisconsin. Keep in mind that when you are located west of the 0° line-of-declination, the declination is east (or positive).
When located east of the 0° line, the declination is west (or negative).
Globally, the magnetic field lines (which make up the magnetic meridian) are similar to the lines of longitude which form the geographic meridian.
That is, they encircle the globe and converge at a common point in each hemisphere. The true magnetic pole in the northern hemisphere presently lies in the
Northwest Territories, Canada. It is about 11.6° south of the geographic north pole, and about 104.3° west longitude.
On a more detailed level however, the magnetic lines are not straight, but bend and arc depending on local magnetic conditions. This bending is
called deviation.
Another factor that alters the value of magnetic declination is called variation, which occurs over long periods of time. Due to the dynamic nature
of the earth's interior, the magnetic field is constantly changing. This causes "drift" of magnetic north and oscillations of the magnetic meridian.
This change in magnetic declination is called annual change (also secular variation). It follows trends in the short term, but is very unpredictable
in the long term. Lines of annual change in Wisconsin now drift about 6.5' west each year. In a year the 0° line will have moved 6.5' west and
by the following year it will be approximately 13' west. (6.5' of westerly shift in Wisconsin is approximately 11 miles).
Using the Internet, you can now determine magnetic declination for any place on the earth since 1900. The National Geophysical Data Center (NGDC) offers two programs for calculating magnetic declination. One is called the Geomagnetic Field Synthesizer Program. There you are prompted to fill in a date and locational information for any part of the world. The program then returns both the horizontal and vertical declination. As a reminder, longitude values in the western hemisphere should be entered as negative.
Search and Rescue Society of British Columbia provides a complete listing of informative map and compass links.
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