Magnetic Declination

How True North Varies From Magnetic North And Why

Close-Up Of Hand Holding Navigational Compass

 

Michael Mhleck / EyeEm / Getty Images 

Magnetic declination, also called magnetic variation, is defined as the angle between compass north and true north at a point on the Earth. Compass north is the direction shown on the north end of a compass needle while true north is the actual direction on the Earth's surface pointing toward the geographic North Pole. Magnetic declination changes based on one's location on the globe and as a result it is very important to surveyors, map makers, navigators and anyone using a compass to find their direction such as hikers. Without adjusting for magnetic declination work done by surveyors could turn out wrong and people like hikers using a compass could easily get lost.

Earth's Magnetic Field

National Geophysical Data Center

Because the Earth's magnetic axis is offset the geographic north and south poles and the magnetic north and south poles are not the same and the difference between these two is magnetic declination.

Magnetic Declination Around the World

According to the Wisconsin State Cartographer's Office, variation inside the Earth "causes a 'drift' of magnetic north and oscillations of the magnetic meridian." The normal change of magnetic declination is called annual change and it is very difficult to predict over long periods.

Finding and Calculating Magnetic Declination

In addition to using a map to find magnetic declination, NOAA's National Geophysical Data Center operates a website that allows users to calculate an estimate of an area's declination via latitude and longitude on a specific date. For example, San Francisco, California, which has a latitude of 37.775̊°N and a longitude of 122.4183̊°W, had an estimated magnetic declination of 13.96̊°W on July 27, 2013. NOAA's calculator also estimates that this value is changing by about 0.1̊°W per year.

When stating magnetic declination it is important to pay attention to whether the calculated declination is positive or negative. A positive declination shows an angle that is clockwise from true north and a negative is counterclockwise.

Using Magnetic Declination and a Compass

In order to adjust for magnetic declination with a map one must find the isoline corresponding with their location or look to the map's legend for a statement of the declination. Magnetic declination calculators such as the one from NOAA's National Geophysical Data Center can also provide this value. Positive declination is then added to orient a compass with a map, while negative declination is subtracted.