The term compass can refer to a navigational tool which points in a fixed direction, allowing travelers to find their way by inferring other directions and paths through the process of orienteering. When used alone the term usually refers to a magnetic compass. The plural compasses can also refer to a device used for drawing circles.
Magnetic compasses rely upon magnetized pieces of metal, typically floating in liquid to ensure low friction. These align themselves with the Earth's magnetic field lines which point at the Earth's magnetic north pole. Magnetic field lines vary from true north (pointing to the geographic north pole) and thus must be adjusted according to information usually found on a map or chart. Maps and charts have a compass rose printed with the deviation from true north noted for the area of coverage of the map. Deviation often occurs due to large mineral deposits which occur in the area of interest.
Deviation from true north can also occur due to the presence of nearby large metal objects, such as ships at sea. At sea, this additional magnetic deviation is tracked, logged, and factored in to reading a magnetic compass in order to determine true north. In an aircraft the aircraft's movement can affect a compass, causing it to give inaccurate readings during turns and other maneuvers.
A gyrocompass, also known as a direction indicator, is another instrument designed to indicate north. They are mainly used in aircraft, where the change in atttude of an aircraft can affect the direction of a normal magnetic compass. The principle of a gyrocompass relies upon a rapidly spinning wheel (usually run electrically) which maintains its original orientation. Due to the law of conservation of angular momentum, a gyrocompass can be set to point to true north when it is initialized, and it will continue to point to true north. Gyrocompasses will drift over time, and therefore need to be resynchronized with a magnetic compass at regular intervals.