Featured Map for 14 November 2014:
Magnetic Variance

Longitude and latitude refer to a coordinate system based on the rotational axis of Earth. The north and south poles are located on this axis, and true north and true south point to these poles.

Earth also has a magnetic field, which has its own magnetic poles. The magnetic poles are not located on Earth's axis, nor are they (usualy) antipodal. At present, the north magnetic pole is about 441 km from the north pole and the south magnetic pole is about 2,868 km from the south pole, but they drift over time and can even flip: over the past 20 million years, Earth has undergone a geomagetic reversal about every 200,000 to 300,000 years.

A magnetic compass points approximately to the north magnetic pole (and away from the south magnetic pole). Approximately, because in addition to the core field there are fields generated in the crust and mantle which vary by location. The difference between true north and magnetic north at any location is known as the magnetic variance or declination and can be measured, or calculated using one of several different models. The variance is important to know for accurate navigation using a magnetic compass.

The Great Circle Mapper now includes the variation on each airport page, and for each path shows the initial magnetic heading in addition to the initial (true) heading. These values are calculated using the World Magnetic Model (WMM).

Today's Featured Map shows today's locations of the magnetic poles, calculated using the International Geomagnetic Reference Field (IGRF11).

References and additional information:

• 2012: Magnetic Pole Reversal Happens All The (Geologic) Time
• National Geophysical Data Center
  • Geomagnetism
  • The World Magnetic Model - Background
  • Wandering of the Geomagnetic poles
  • The World Magnetic Model
  • International Geomagnetic Reference Field