magnetic heading

[ElNull]

would someone please explain to me why at the polar regions magnetic heading should be considered unreliable and useless?

[Nimer767]

When your over either the north pole, or the magnetic north pole, you will be flying what is called (Grid Tracks) Simplistically, it's a track based navigation, opposed to heading based system. in other word TRUE HEADING

Because the magnetic poles and geographic poles are located in different places, the magnetic compass becomes somehow useless in the far northern regions

[Northbeach]

ElNull,

As you know the geographic North Pole is not located in the same place as the magnetic North Pole. Additionally the magnetic North Pole is in almost constant motion [magnetic North Pole is moving, albeit very slowly].

In the extreme case if you were physically standing between the geographic North Pole and the magnetic North Pole [facing the geographic North Pole] your compass would point in one direction to the magnetic North Pole while the geographic pole is in the opposite direction. Consequently in the far Polar Regions the magnetic compass becomes more problematic.

Most maps are laid out with true north at the top of the map. A course drawn on such a map is in relation to true north. However the most readily available instrument that most of us have [until recently] is the compass which points not to true north but magnetic nroth. So whatever you read from a compass is a compass course, yet the map is related to true north.

The first correction made is usually for the difference between magnetic north and true north. I was taught that this difference is called “variation”. One adds westerly variation and subtracts easterly variation to arrive at “Magnetic Course”. This would be the course you would follow on your compass [if it were perfectly accurate] to travel the line you drew on the map. But we are not done, because most compasses are not perfectly accurate.

Most compasses have errors associated with the compass instrument itself caused by the location of magnetic material mounted physically close to the compass in an aircraft or vessel. This would include the metal in the surroundings, radios, instruments, electrical current present or the metal casing surrounding the compass itself. This magnetic interference is called “deviation”. Most compasses have a “compass correction card” permanently located near the compass that gives the operator the various corrections for every 10 degrees.

Beginning with your true course you corrected for magnetic variation to arrive at your magnetic course. You corrected your magnetic course for deviation to arrive at your compass course.

In your original post you used the word heading instead of course. Once you correct for wind drift at altitude you have a heading instead of a course. Consequently a course drawn on a map corrected for magnetic variation and deviation and winds aloft would become a compass heading.

Respectfully,

Northbeach

[Exaviator]

Whilst Northbeach has explained the difference between True & Magnetic North the reason steering by a magnetic compass in the high latitudes becomes impractical is due to the convergence of Longitude which in theory becomes zero at the True North Pole. The difference between True & Magnetic North becomes magnified as the Longitude converges. Using a magnetic compass to steer would result in a constantly changing heading as the card rotates to align with the Magnetic Pole.

Compass dip also becomes a factor as you get closer to the poles. :ok:

[LH2]

While the facts in the answers above are largely correct, they are not by themselves what makes nearly impossible the use of compass in high latitudes.

Exaviator has, on the other hand, pointed out the actual reason: as you approach the magnetic poles, the inclination (i.e., the angle between the magnetic flux and the local horizon--aka compass dip) becomes nearly vertical, meaning that the compass needle points down into the Earth which is not terribly useful for navigation.

This schematic (http://www.erikdeman.de/pictures/pic103.gif) pulled via Google Images should give you a visual idea of why it doesn't work.