<P> Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century . Although the magnetic declination does shift with time, this wandering is slow enough that a simple compass remains useful for navigation . Using magnetoreception various other organisms, ranging from some types of bacteria to pigeons, use the Earth's magnetic field for orientation and navigation . </P> <P> At any location, the Earth's magnetic field can be represented by a three - dimensional vector . A typical procedure for measuring its direction is to use a compass to determine the direction of magnetic North . Its angle relative to true North is the declination (D) or variation . Facing magnetic North, the angle the field makes with the horizontal is the inclination (I) or magnetic dip . The intensity (F) of the field is proportional to the force it exerts on a magnet . Another common representation is in X (North), Y (East) and Z (Down) coordinates . </P> <P> The intensity of the field is often measured in gauss (G), but is generally reported in nanoteslas (nT), with 1 G = 100,000 nT . A nanotesla is also referred to as a gamma (γ). The tesla is the SI unit of the magnetic field, B . The Earth's field ranges between approximately 25,000 and 65,000 nT (0.25--0.65 G). By comparison, a strong refrigerator magnet has a field of about 10,000,000 nanoteslas (100 G). </P> <P> A map of intensity contours is called an isodynamic chart . As the World Magnetic Model shows, the intensity tends to decrease from the poles to the equator . A minimum intensity occurs in the South Atlantic Anomaly over South America while there are maxima over northern Canada, Siberia, and the coast of Antarctica south of Australia . </P>

What is the value of earth's magnetic field