How to Read a Mariner's Compass

Reading a mariner compass may look complicated, but don't let all those intricate pieces throw you off. Mariners have been using compasses based on the same principle design for centuries. Even in this day of satellite imagery and echolocation, the compass remains one of our most efficient and reliable navigation devices.

Compass Definition

The compass definition is a device that uses a magnetized pointer in order to show you the direction of magnetic North. Colloquially, we say that a compass is a tool that shows you what direction North is in based on your location.

A compass works by using the natural magnetism in the Earth. All magnets, including the Earth, have two distinct poles: the north pole and the south pole. You might recognize those words based on locations on our planet, which actually got their names because of the magnetic field of the Earth.

Other magnets want to properly align with larger magnets and magnets around them. Since the Earth is the largest magnet around, your magnetic compass uses magnetic compass navigation; the needle will align itself with the magnetic pole of the Earth, which causes your pointer to point you towards Earth's north.

Before You Set Sail with Your Compass

    Take your mariner compass to a professional to have it "zeroed," or adjusted for deviations that can arise due to the presence of metals in your vessel's structure.

    Learn the parts of your magnetic compass navigation. The "bowl" is the liquid-filled casing that contains the "compass card," a magnetized piece with an arrow that always points toward "magnetic north." While "true north" refers to the Earth's North Pole, "magnetic north" refers to a point several miles to the east.

    The bowl is mounted on the "baseplate." The ring around the bowl is called the "central dial." On the central dial, you will see a mark for the "lubber line." Your lubber line will vary as you rotate the dial.

    Make sure that the area around your compass is free of iron and magnets, which may affect your reading. Align "north" on the dial with the north end of the needle on the compass card, which is usually printed in red.

At Sea

    Calculate your "variation," which is the angle between true north and magnetic north, by referring to the mariner compass rose on your chart. Magnetic north changes gradually over time (and has since the Earth's initial formation) so adjustments are made every year; always keep current charts to have a more accurate reading.

    Align your lubber line with the vessel's keel. Depending on your magnetic compass navigation card, match the angle you obtained in Step 1 of Section 2 with the angle displayed on either the front or rear of your compass card. If you have a flat card, your heading will be displayed closest to the bow; if your card protrudes and headings are marked around the outside of it, the heading will be the number closest to the stern.

    Maintain the heading as steadily as possible for 1 mile. At the mile mark, readjust your heading; the angle from true north to magnetic north will have changed.

    Warnings

    • A compass is not generally as accurate as a working GPS system. However, while a compass is considered an effective fallback tool for a GPS system, a GPS system should not replace the compass on a seagoing vessel. A compass does not depend on batteries or electricity, making it more reliable than a GPS system. While you can engage in nautical travel without a GPS system, don't ever travel without a compass.

References

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