As stated in the topic on magnetism, Earth has magnetic properties and
can be thought of as having a powerful magnetic bar near its center.
The lines of force appear as illustrated in figure 2-3.
Figure 2-3. Chart No. 42 showing lines of variation.
Earths magnetic properties are not uniformly distributed.
Earths magnetic poles are not at the same position as the geographic
Magnetic lines of force are called magnetic meridians.
The variation for any area on
Earth is always equal to the
difference between the value of
true north and magnetic north.
Figure 2-4. Variation equals the
angular difference between true north
and magnetic north.