Reading a 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.
Before You Set Sail
Take your 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 compass. 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.
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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.
Calculate your "variation," which is the angle between true north and magnetic north, by referring to the compass rose on your chart. Magnetic north changes gradually over time, so adjustments are made every year; always keep current charts.
Align your lubber line with the vessel's keel. Depending on your compass 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.
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.