COMPASS Points and Headings

Useful marine navigation information about
compasses and some electronics from Mainstay Systems, Inc.

Today most boats have one or more magnetic compasses. Many boats also carry one or more electronic (fluxgate or similar) compasses. Some larger vessels carry a gyrocompass. And increasingly more vessels are using GPS satellite navigation receiver equipment to provide "course" information, even though a GPS does not have a compass. The following paragraphs offer some thoughts on marine compass use. I welcome your insights and comments.

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MAGNETIC Compass:

ADVANTAGES:

• Does not require any electrical power to function (except for night illumination), therefore is not affected by loss of power.
• Tends to be more stable than a fluxgate compass. Usually, the bigger the compass card diameter on a magnetic compass, the more stable, (and also the easier to read).
• Depending on design and location, may be used to take bearings, as well as to display vessel's heading.
• Heading display is analog, preferred by many mariners.
• Can be compensated for magnetic deviation by the use of adjusting magnets.
• History of reliability.

DISADVANTAGES:

• Lack of digital output make compass inappropriate for interface with electronic navigation devices (although some magnetic compasses can be fitted with special electronic pickoffs to interface with an autopilot).
• Compass usually must be located at the steering station for reading by the helmsman. It can not be located remotely in a low magnetic interference area (ie: under a sleeping bunk).
• Second (backup) magnetic compass cannot be located close to the primary magnetic compass. The two compasses will interfere with each other, making both inaccurate.
• Magnetic compasses sometimes can be rendered inaccurate by a lightning strike on the vessel, or by arc welding in the close vicinity of a magnetic compass (compass should be temporarily removed while welding in the vicinity).
• Must be corrected for magnetic variation, which changes with location on the earth.

ELECTRONIC (Fluxgate or Similar) Compass:

ADVANTAGES:

• Digital output easily interfaced with other electronic navigation equipment - autopilot, radar, chart plotter can use heading information beneficially.
• Compass sensor can be located remotely and away from strong magnetic influences, such as under sleeping berth.
• More than one digital course indicator, fed from one compass sensor, can be mounted in other locations.
• Can be compensated for magnetic deviation and incorrect alignment, often electronically at the operating console.

DISADVANTAGES:

• Loss of electric power renders the compass dead.
• As with any electronic equipment, subject to failure as a result of a bad electronic component.
• Often less stable than a good quality magnetic compass, especially in rough sea conditions.
• Must be corrected for magnetic variation, which changes with location on the earth (sometimes magnetic variation can be programmed into an associated computer, to make this correction automatically, making the compass read TRUE heading.

GYROCOMPASS:

ADVANTAGES:

• Provides TRUE heading. Does not have to be corrected for magnetic variation.
• Can be located remotely or at the steering station.
• Can feed heading readings to one or more remote readouts.
• Can be interfaced to other navigational equipments.
• Is not affected be magnetic influences on the vessel.

DISADVANTAGES:

• Far more expensive to purchase and install than magnetic or fluxgate compasses.
• Requires expensive professional periodic routine maintenance for continued reliability.
• Subject to inoperability due to mechanical, electrical, or electronic component malfunction.
• Repairs require expensive, sometimes not locally available professional help.

GPS (Global Positionong System) Course:

GPS is a satellite navigation system, developed by the U.S Navy, which has recently become quite popular in commercial and recreational watercraft. The system provides PRIMARILY accurate position (Latitude and Longitude) of the receiving station. GPS is electronic and has no compass of any kind. Usually the GPS receiver is programmed to calculate a course of the vessel from a series of positions. The operator can select TRUE or MAGNETIC course. For the course to be of any accuracy and value, the GPS receiver must be locked on and receiving accurate satellite information, and the vessel also must be traveling in a straight line (when the vessel is tied to a dock, anchored or otherwise not moving, any course provided by GPS is meaningless). The navigator must realize, when using GPS course information, that the GPS course can be quite different from the compass course. GPS course is "course over the ground", whereas compass course is "course through the water," also known as "heading". When in a cross wind or current, especially at the low speeds at which many boats travel, the "set and drift" will be included in the calculated GPS course, but will not be known to the compass. (A sailboat crossing the Gulf Stream at about seven knots may easily experience 15 or more degrees difference between heading and GPS course).

© Marvin L. Schenker 1996

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GPS Operations Information - US Navy
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