What you may not know about your compass could get you lost!

Compasses are instruments for determining horizontal direction for navigation over land, in the air and on water. There are several types of compasses in common use. Each has a specific use for which it is best. All are affected by several basic principles that you must take into account in order to rely on their accuracy. Many mariners don't think a compass is necessary with GPS freely available and providing a high degree of accuracy. Having been in situations where we have lost all power, including battery backup, as well as heavy weather situations where I could not see to the chartplotter from the wheel, I know the compass is one of my best friends. It almost never fails; its only failure is if I have improperly corrected for the factors that can affect it's performance, namely variation and deviation!

Types of Compasses

Marine compass (Card compass).

In this compass the needle is fixed while the compass card is mounted in fluid and moves. Because the moving card absorbs much of the motion of a boat it is easier to read than a needle compass. The top of the compass is typically a hemispherical shape and may provide magnification of the readings. The card is normally marked to show all readings from 0° through to 360°. These compasses are usually mounted on the boat near to the steering apparatus in which case they are called the steering compass. Care must be taken not to allow nearby magnetic fields (e.g. metal and electronic objects) to interfere with the working of the compass.

Gyrocompass.

This is an electric compass and as such is not affected by magnetic fields. Readings are true bearings and there is no need for adjustment for the Earth's magnetic field. These compasses are sophisticated, stable and very accurate. Large ocean-going vessels use a gyrocompass as their steering compass. Due to their size, weight and cost they are not usually found in use on smaller craft.

Hand held compass

Hand held models can be used to sight bearings of objects in the distance when fixing position. For marine use, they typically have a moving card suspended in fluid much like the marine compass, and will include a sight to line up with objects in the distance. Some binoculars also integrate stabilized compasses to sight a fix. For more on use of hand-bearing compasses, click here.

Needle compass (Pocket compass).

This type of compass has a magnetised needle suspended over a fixed card which has been marked off in a clockwise direction in 360 equal units (degrees). It's the kind used by hikers and boy scouts. The cardinal compass points, north (0°), east (90°), south (180°) and west (270°) are always clearly marked. Intercardinal points (NE, SE, SW and NW) are usually shown plus divisions between these and the cardinal points, thus giving a total of sixteen compass directions. The needle moves freely to align itself north-south as the compass is turned.

Prismatic compass.

This compass has a glass prism sighting arrangement and a lid with a hairline for lining up the object to be sighted. The compass card rotates in the base and when it comes to rest the required bearing is read off, through the prisms. These are quite sophisticated hand compasses.

Baseplate compass (Orienteering compass).

This type of compass was invented in the 1930s in Sweden. It saved a lot of time transferring bearings from compass to map. It proved to be a greatly successful system of direction-finding for outdoor activities. These compasses are made of clear plastic. They have a rectangular base with a 360° dial mounted upon it. Inside the raised dial is a magnetic needle suspended in clear fluid. The dial may be turned to read a correct bearing along the direction of travel arrow which is clearly marked on the base plate.

Thumb compass.

This is a recent (1980s) modification of the baseplate compass where the baseplate is re-modelled to fit around the thumb so that the map and compass can be held together in one hand, leaving the other hand free. It is used mainly for orienteering.

Sun compass.

This is non-magnetic and uses the path of the sun as a reference line to indicate direction and works in reverse to a sundial.

Astrocompass.

This is similar in principle to the sun compass but can be used with any celestial body. A frequent use is in the navigation of aircraft.

Gyrosyn compass.

This composite instrument, using a magnetic compass stabilised by a gyroscope, is able to adjust quickly for changes in course and for this reason is widely used in aircraft.

Did you Know?

A "dead beat" compass is one in which the mechanical design, including the moving mass, resistance and fluid type, have been arranged in such a way that when an outside influence diverts the card (or needle) from north, it will swing back to north and settle on north WITHOUT overshooting and oscillating before settling. The card is settled regardless of stormy conditions it is a 'dead beat'! "Dead beat" is a clock maker's term and it refers to a type of escapement mechanism. It was adopted by the early makers of fluid filled compasses to describe the effect explained above.
Capt. Chris Bingham (British commercial sea captain, ret'd).
A "dumb" compass is another name for a pelorus. A pelorus consists of a circular metal plate with a compass card and alidade mounted on its surface, each of which may be revolved independently of the other. It is used for determining the horizontal angle between the vessel's heading and the bearing of an observed object, especially the bearings of objects which are not visible from the steering compass.