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Compass

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Several different types, such as the Silva compass, Prismatic compass (sometimes known as the Marching compass), Ship's Compass and Gyrocompass.


All have the ability to indicate the direction of the North Pole, usually by means of a magnetised needle (except the Gyrocompass) and thus are highly useful for Map Reading and navigation.


Their limitations are that they can be confused by stray magnetic fields such as those generated by power cables, electrical machinery or metal objects, and the Grid Magnetic Angle varies with mapping scheme sheet and region.


The Gyrocompass overcomes these limitations by using a gyroscope instead of a magnetized needle so they can be used on a ship or aircraft where it's impossible to move a normal compass far enough away from the metallic bits to prevent interference.

The downfall of the Gyrocompass is that it requires a power supply and it's feckin' expensive.


Before the advent of the Gyrocompass, a normal magnetic compass was used, calibrated or "swung" to the vessel it was mounted in.