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What is declination?

At most places on the Earth's surface, the compass doesn't point exactly toward geographic north. The deviation of the compass from true north is an angle called "declination" (or "magnetic declination"). It is a quantity that has been a nuisance to navigators for centuries, especially since it varies with both geographic location and time. It might surprise you to know that at very high latitudes, the compass can even point south!

The collar of USGS topographic maps shows the magnetic declination at the center of the map the year that the map was made. That's important information for anyone who is using the map and a compass to navigate. NOAA has an online calculator for estimating the declination at any longitude/latitude on a specific date.

Declination is simply a manifestation of the complexity of the geomagnetic field. The field is not perfectly symmetrical; it has non-dipolar "ingredients," and the dipole itself is not perfectly aligned with the rotational axis of the Earth. If you were to stand at the north geomagnetic pole, your compass, held horizontally as usual, would not have a preference to point in any particular direction, and the same would be true if you were standing at the south geomagnetic pole. If you were to hold your compass on its side, the north-pointing end of the compass would point down at the north geomagnetic pole, and it would point up at the south geomagnetic pole.

The USGS Geomagnetism Program operates magnetic observatories in more than a dozen locations around the United States.

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