The strength and direction of the magnetic field describe a vector. The coordinate system used to
order the magnetic field vector is usually one of two types -
- and is usually orientated in one of two ways -
Figure 1b shows a local cylindrical polar coordinate system in which the magnetic field vector, F, has components, H, D, and Z. Here Z is the vertical component defined as positive downwards, as before, H is the component of the magnetic field in the horizontal plane, and D is the angle of the horizontal field component, H, from the eastward direction. D is referred to as the declination. (It is often to be seen on maps, indicating the angle between geographic and geomagnetic north.) Another angle that may be quoted is also shown in the figure. It is the inclination angle, I, between the vector, F, and the horizontal plane. We can see that (F, /2 -I , D) define a spherical polar coordinate system.
The various coordinate systems are often distinguished by using different letters for their orthogonal axes. For example, it is common to use (X, Y, Z) for a geographic coordinate system and (H, D, Z) for a geomagnetic coordinate system. However, there is no set standard and so any rules are made to be broken. For example, some people may prefer to use (X, Y, Z) and ( H, D, Z) to distinguish between Cartesian and cylindrical polar coordinates, respectively. You have been warned! At BAS we use (H, D, Z) to define a geomagnetic Cartesian coordinate system.
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