Magnetic Declination


	Magnetic Declination

(What do you mean "North isn't North"?)

Many people are surprised to learn that a magnetic compass does not normally point to true north. In fact, over most of the Earth it points at some angle east or west of true (geographic) north. The direction in which the compass needle points is referred to as magnetic north, and the angle between magnetic north and the true north direction is called magnetic declination. You will often hear the terms "variation", "magnetic variation", or "compass variation" used in place of magnetic declination, especially by mariners.

The magnetic declination does not remain constant in time. Complex fluid motion in the outer core of the Earth (the molten metallic region that lies from 2800 to 5000 km below the Earth's surface) causes the magnetic field to change slowly with time. This change is known as secular variation.Because of secular variation, declination values shown on old topographic, marine and aeronautical charts need to be updated if they are to be used without large errors. Unfortunately, the annual change corrections given on most of these maps cannot be applied reliably if the maps are more than a few years old since the secular variation also changes with time in an unpredictable manner.

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. This point in the northern hemisphere presently lies in the Northwest Territories, Canada: the true magnetic pole 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.

The Magnetic Compass

The magnetic compass has been used for navigation for hundreds of years. At one time, it was the only reliable means of direction-finding on days when the sun and stars were not visible. Nowadays, sophisticated equipment is available that enables users to determine their bearing accurately and to pinpoint locations to within a few metres. However, such equipment has not made the compass obsolete. It is still a very practical tool for navigation for many small craft and for people on foot. Even airplanes and ships equipped with more sophisticated equipment often carry compasses as backups.

Regardless of their intended purpose or the complexity of their construction, most compasses operate on the same basic principle. A small, elongated, permanently magnetized needle is placed on a pivot so that it may rotate freely in the horizontal plane. The Earth's magnetic field which is shaped approximately like the field around a simple bar magnet exerts forces on the compass needle, causing it to rotate until it comes to rest in the same horizontal direction as the magnetic field. Over much of the Earth, this direction is roughly true north, which accounts for the compass's importance for navigation.

Area of Compass Unreliability

The horizontal force of the magnetic field, responsible for the direction in which a compass needle is oriented, decreases in strength as it approaches the North Magnetic Pole, where it is zero. Close to the pole, an area is reached where the frictional forces in the pivot are comparable to the horizontal forces of the magnetic field. The compass starts to behave erratically, and eventually, as the horizontal force decreases even more, the compass becomes unusable.

Magnetic Reference Field Models

Since magnetic observations are neither uniformly nor densely distributed over the Earth, and since the magnetic field is constantly changing in time, it is not possible to obtain up-to-date values of declination directly from a database of past observations. Instead, the data are analyzed to produce a mathematical routine called a magnetic reference field "model", from which magnetic declination can be calculated. Global models are produced every five years. These constitute the series of International Geomagnetic Reference Field (IGRF) models. The latest IGRF was produced in 1995, and is valid until 2000.

Since magnetic field models such as the IGRF are approximations to observed data, a value of declination computed using the model is likely to differ somewhat from the "true" value at that location. It is generally agreed that the IGRF achieves an overall accuracy of better than 1° in declination; the accuracy is better than this in densely surveyed areas such as Europe and North America, and worse in oceanic areas such as the south Pacific. The accuracy of all models decreases in the Arctic near the North Magnetic Pole.

Magnetic Declination Calculation

When using magnetic declination values, keep in mind that when you are located west of the 0° line-of-declination, the declination is east, whereby you must subtract the declination value from the calculated azimuth value. When located east of the 0° line, the declination is west, whereby you must add the declination value to the calculated azimuth value.

To assist your azimuth settings, we have provided a Magnetic Declination Map. Find your location and refer to the declination amount. You will need to add or subtract the declination amount from the true azimuth value you calculated with the dish alignment utility.

References:

National Geophysical Data Center[add link]
World Data Center A for Solid Earth Geophysics [add link]
Geological Survey of Canada