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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ELECTRONIC (Fluxgate or Similar) Compass:






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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Course Conversion Calculator
Chapman Seamanship School
GPS Operations Information - US Navy
Magnetic Variation Info
NOAA On-Line Chart Viewer
U S Geomagnetic Survey


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USA Today Weather
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National Oceanic and Atmospheric Administration (NOAA)


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