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Magnetic
Variation
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Introduction to Glider Flying > Flight Instruments > Magnetic Compass > Magnetic
Variation
Although the magnetic field of the Earth lies
roughly north and south, the Earth’s magnetic poles do
not coincide with its geographic poles, which are used in the
construction of aeronautical charts. Consequently, at most places
on the Earth’s surface, the direction-sensi-tive steel
needles, which seek the Earth’s magnetic field, will not
point to True North but to Magnetic North. Furthermore, local
magnetic fields from mineral deposits and other conditions may
distort the Earth’s
magnetic field and cause an additional error in the posi-tion
of the compass’ north-seeking magnetized needles with
refer-ence to True North. The angular difference between True
North and the direction indicated by the magnetic com-pass—
excluding deviation error—is variation. Variation is different
for different points on the Earth’s surface and is shown
on the aeronautical charts as broken lines con-necting points
of equal variation. These lines are isogonic lines. The line
where the magnetic variation is zero is an agonic line. [Figure
4-16]
Figure 4-16. Earth’s magnetic
field.
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