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Geomagnetic models form the foundation of traditional, compass-based navigational systems. These models provide a picture of the Earth's magnetic field and how it varies from one point on the Earth's surface to another.

The primary world model is the International Geomagnetic Reference Field (IGRF), compiled from magnetic measurements collected by national observatories in many countries, as well as readings made from ships, airplanes, and satellites. The model, derived through mathematical analysis of a vast amount of data, represents the magnetic field generated in the Earth's core, with small-scale variations at the surface and solar effects filtered out of the basic data. Even in an age of Global Positioning System (GPS) navigation, when finding your position on the Earth's surface is just a click away, the geomagnetic model still plays a vital role, it is built into GPS navigation systems as a backup. The geomagnetic field model is also vital to various kinds of magnetic surveys, such as those used in mineral exploration and the mapping of hazardous earthquake faults.

Diagram courtesy of Scientific American (December 1989) The Earth's magnetic field is generated within its molten iron core through a combination of thermal movement, the Earth's daily rotation, and electrical forces within the core. These elements form a dynamo that sustains a magnetic field that is similar to that of a bar magnet slightly inclined to a line that joins the North and South Geographic Poles. A compass placed in this magnetic field thus does not point due north, declination measures the angle between the compass reading at any point on the Earth's surface and true north (measured in degrees). The geomagnetic reference model is the basis for establishing the declination and its variation across the surface of the globe.

The direction and strength of the magnetic field can be measured at the surface of the Earth and plotted. The total magnetic field can be divided into several components:

• Declination (D) indicates the difference, in degrees, between the headings of true north and magnetic north.
• Inclination (I) is the angle, in degrees, of the magnetic field above or below horizontal.
• Horizontal Intensity (H) defines the horizontal component of the total field intensity.
• Vertical Intensity (Z) defines the vertical component of the total field intensity.
• Total Intensity (F) is the strength of the magnetic field, not divided into its component parts.

Courtesy of John Hillhouse, USGS The intensity and structure of the Earth's magnetic field are always changing, slowly but erratically, reflecting the influence of the flow of thermal currents within the iron core. This variation is reflected in part by the wandering of the North and South Geomagnetic Poles. Because a wide range of commercial and military navigation and attitude/heading systems are dependent on models of the magnetic field, these models need to be updated periodically. The magnetic field's strength and direction and their rates of change are predicted every 5 years for a 5-year period.