When the angle is expressed with relation to true north, it is known as True Heading
(TH). When it is measured from magnetic north, it is called Magnetic Heading (MH). If it
is measured from compass north, the term would be Compass Heading (CH). In each
case, the angle is measured in a clockwise direction from the north reference to the
longitudinal axis of the aircraft. You should determine which reference was used, since
each reference has a different value, which is why it is important to specify true,
magnetic, or compass as shown in Figure 2-1.
Figure 2-1 -- Designating compass heading.
When a magnetized needle is influenced by earth's magnetic field, the direction it points
is magnetic north. The direction of the geographic North Pole is called true north. The
angle between magnetic north and true north is termed variation. Variation differs at
different points on earth. When the needle points to true north, then magnetic north and
true north coincide and the variation is zero. When the needle points east of true north,
the variation is east; when the needle points west of true north, the variation is west.
A compass, however, is affected by all magnetic fields. A piece of iron close to a
compass needle tends to deflect it from magnetic north. Whenever an electric current
passes through a wire, a magnetic field is set up around the wire. The combined effect
of all the magnetic fields within the aircraft causes deviation.
Deviation varies as an aircraft changes headings because the metal structure and
electrical devices turn with the aircraft, creating a different alignment relationship.
Since deviation may vary with each heading, deviation is determined for each heading
that differs by approximately 15 degrees. This is done most commonly on the ground
with a large compass rose (a large concrete area) with magnetic headings inscribed at
15-degree increments. Comparing the compass reading to the known magnetic heading
yields the deviation. If deviation is present and the north point of the compass points