FAO Home>Fisheries & Aquaculture
Food and Agriculture Organization of the United Nationsfor a world without hunger
Equipment Class: Fishing technologyEquipment Type: Wheelhouse
Compass  in navigation or surveying, is the primary device for direction-finding on the surface of the earth. Compasses may operate on magnetic or gyroscopic principles or by determining the direction of the Sun or a star.
A compass (the SP-5 Globemaster)
A compass (the SP-5 Globemaster)
Courtesy of Ritchie

The oldest and most familiar type of compass is the magnetic compass, but nowadays other more sophisticated compasses are available, like gyrocompass and fluxgate compass.

The Magnetic compass, as the name implies, relies on the earth's magnetic field and, because the true poles and the magnetic poles on the earth's surface do not coincide, there are various corrections that have to be applied. Magnetic compasses have been developed over hundreds of years and various means have been used to optimize their performance. Magnetic compasses are still retained because of their independence from electrical power. Nevertheless, for magnetic compasses required for emergency purposes there is a continuing need for regular compass adjustment and care in ensuring that metal objects are not placed in the vicinity of the compass, which would be adversely affect it.

Most larger ships carry a gyrocompass which relies on the gyroscopic effect in which a freely spinning body aligns itself at right angles to the direction of movement. The gyroscope senses the spinning movement of the earth and aligns its axis of spin north/south. The advantages of the gyroscope is that it does not require corrections of the magnetic compass but it does require a reliable source of electricity and also takes some time to start up.

On smaller vessels, fluxgate compasses have become very popular, but again they require a reliable electrical source although they do have digital capabilities.

While at sea, the direction in which a vessel is moving is measured by a compass. By convention, most compasses are now graduated in terms of 360 degrees. However, some earlier magnetic compasses are graduated with the points and quarter points of the compass. The importance of the compass in vessel navigation is recognised in its central position where the helmsman can see it at all times.
As more electronic equipment has been introduced to the bridge of fishing vessels there has been a need to interface the compass with other instrumentation, such as navigation plotters and autopilots. Generally this is done in conformity with NMEA 0183 protocols which establish the method by which communication are established between two pieces of electronic equipment. Note that because a magnetic compass is not electronic, an electronic sensor must be linked to the compass to provide an electronic output of the vessel's heading.

Powered by FIGIS