The answer comes from Paul Perreault, marketing manager for university and government research programs at Trimble Navigation Ltd., a manufacturer of advanced navigation systems.
Image: Canadian National Geomagnetism Program
The North magnetic pole is at a point where a dipping compass--a compass that allows the needle to move freely in a vertical plane (as opposed to the horizontal needle movements seen in most compasses)--points straight down into the earth. The South magnetic pole is the point where a dipping compass points up. A dipping compass points horizontally on the Earth's magnetic equator, also called the Earth's dip equator.
The magnetic poles are quite distant from their geographic counterparts. The North magnetic pole is located to the south in Northern Canada; the geographic South pole is at the center of the Antarctic continent, but the magnetic pole is hundreds of miles away, near the coast. In regions near the magnetic poles, compasses are virtually useless.
Complicating this issue is that these pole positions are not static--for either magnetic or geographic poles. The location of the North geographic pole wanders in a small erratic circle-like path, called the "Chandler wobble." This motion is less than 6 meters per year on the surface; a worldwide network of very precise global positioning satellite (GPS) receivers is used to determine this wander.
The magnetic poles are far more restless. They move under the influence of the dynamo currents in the Earth's core, as well as electric currents flowing in the ionosphere, the radiation belts and the Earth's magnetosphere. The North magnetic pole seems to be moving northward at an average rate of 10 kilometers per year. Yet there is also some elliptical motion to this general trend. On any given day, the magnetic pole may be as much as 80 kilometers away from its average position, depending on the geomagnetic disturbances in the ionosphere and magnetosphere.
Modern navigators normally are not affected by the wandering of the poles because they can regularly determine their position from satellites and Earth-based observatories. The degree of difference between the position of these two poles when seen at various locations is called magnetic declination. These angles allow navigators to determine their actual geographic position. Charts and handbooks for navigation regularly update these values and are published by government agencies.
Maritime and air navigation regulations require use of these up-to-date charts and tables. Without GPS, the navigator would use a new version of the chart or table. With GPS, navigators update their receiver database by purchasing new data from the Government or from the GPS manufacturer and loading it into the onboard GPS unit.