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Chapter 7- Navigation for Divers Navigation for Divers 199 Compass Navigation A compass provides a navigational reference even when natural aids are not available. The magnetic needle of a compass aligns itself with the earth’s magnetic poles. Magnetic north is a point in extreme northern Canada towards which a magnetic needle points. True north is a geographical location with reference to the earth’s axis rather than to the magnetic poles and is not the same as magnetic north. Compass declination, or variation, is the local differences in degrees between true, map or chart, and magnetic north. Fortunately, diving navigation involves relative direction rather than true direction, so the effects of variations do not pose problems for divers. Compass deviation occurs when the needle of a compass is attracted by nearby metal or a magnetic source, such as another compass. Deviation can cause errors in compass readings for divers, so you must be aware of its effects and prevent any deviation. Usually a metal object must be within three feet of a compass to cause deviation to occur. This can be tested with your diving equipment by placing your diving compass on a floor, noting the needle reading, then passing your other equipment past the compass and seeing if deviation occurs and, if so, at what distance. The steel cylinder is most likely to have an effect. A popular type of diving compass is the indirect needle type with index markers on a rotating bezel. This section explains how to use this type of compass. Manufacturer’s instructions will explain how to use other types. No matter which type of compass you select, there are procedures for the use and care of a compass with which you need to be familiar. To set a heading with your compass, point the lubber line of the compass in the desired direction of travel and align the index marks so they bracket the north end of the compass needle. To follow a heading, simply keep the lubber line aligned with your body and keep the compass needle between the index marks. Keeping the compass aligned with the centerline of your body is very important. If the compass is worn on the wrist, good alignment can be obtained by fully extending the arm without the compass and grasping the elbow joint of the extended arm with the middle finger of the arm equipped with the compass (figure 7-8). An even more accurate way to align the compass is to hold it in both hands extended in front of the diver. A console-mounted compass can be aligned by holding the console in both hands with the elbows tucked against your sides (figure 7-9). This method is not as accurate as the two other positions described. The most accurate way to use a compass is to mount it on a compass board that is held in both hands and extended in front of the diver. The extended lubber line on the compass board helps reduce any error in the ways the compass is used. You should sight across a compass when following a heading rather than looking down on the instrument. As you look across the compass, select an object ahead Antenna Telephone Pole Dive Site Fence Post Edge of House FIGURE 7-7. TWO SETS OF IN-LINE OBJECTS ESTABLISH A FIX FIGURE 7-8. DIVER NAVIGATION UNDERWATER WITH A WRIST-MOUNTED COMPASS


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