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Magnetic North is the direction of the horizontal component of the earth's magnetic field. Except near the poles, this direction is approximately true north. The angle between the direction of local Magnetic North and True North is called variation (also declination). It varies from place to place and also slowly in time, and is charted on both VFR and IFR navigation charts. ``East'' variation means that magnetic North lies to the East of true North, so that directions are related by:

\hbox{Magnetic Direction = True Direction (-E/+W) Variation}
\end{displaymath} (1)

as illustrated in Figure 1.

Figure 1: Relation of True and Magnetic headings

The fact that even ideal magnetic compasses reference direction to magnetic North should not be thought of an error. Nevertheless it needs to be accounted for if one is interested in obtaining true headings.

Ed Williams