An isogonic chart. Magnetic meridians are in black and
geographic meridians and parallels are in blue.
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There are two types of poles on the Earth: two geographic
poles and a magnetic pole. The geographic poles are located at the exact
northern and southern-most points on the globe. The North Pole is at 90º N
latitude, and the South Pole is at 90º S. All lines of latitude come together
(or converge) at the north and south poles. This is also called true north and
south.
A compass will point to the magnetic north pole. This
is a point in northern Canada located close to 71º N latitude, 96º W that is
about 1,300 miles (2,092 kilometers) from true north. It is where the Earth's
magnetic pull is the strongest. The lines of force that lead to the magnetic
pole are not straight like the lines that lead to the geographic poles but
weave back and forth according to the magnetic fields in the Earth.
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A typical isogonic chart. The black lines are isogonic lines
that connect geographic points with identical magnetic variation. An agonic
line depicts a point where there is no variation between true north and
magnetic north.
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When a pilot plots his course, he must allow for the
difference between the degrees as shown on a compass and the degrees as shown
on a geographic map. The angle between a magnetic and a geographic meridian
(the lines of longitude) is called variation. For instance, if the variation is
shown as 9º E, this means that magnetic north is 9º east of true north. The
type of map or chart that connects the points with the same magnetic variation
is called an isogonic chart. |
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