Chapter 7- Navigation for Divers Navigation for Divers 197 following formula: D = ST D = 0.4 m (1.25 ft)/sec. x 140 secs. = 56 m (175 ft) Currents can invalidate data gathered from any of these techniques. Be sure their effect is taken into account if they are present. When visibility is limited, when the situation requires starting and stopping, or when very accurate distance measurements are required, arm spans may be used. The exact length of the span of the arms, measured with the arms extended in line with the body rather than perpendicular to the body, is needed. The arm span measurement will be approximately equal to your height. You can then measure distance underwater by extending one arm backward and placing the fingertips on the bottom, extending the other arm forward in the direction of travel, moving forward while pivoting on the fingertips of the extended hand and keeping that hand stationary, then bringing the trailing hand forward, etc. The distance can be measured by multiplying the length of the arm span times the number of spans required. A very accurate means of distance measuring underwater is with a measured line. One end of the line is secured to a stationary object or held by your buddy while you unreel the line and swim the distance to be measured. Do not use line that stretches or your measurements will be inaccurate. This is the only means of accurate measurement if currents are present and variable. MEANS OF NAVIGATION You may navigate by using natural aids to navigation, by using instruments for reference, or by using all of these in combination. When your location is confirmed with visual check points, you are navigating by pilotage. When your location is an estimation based on distance and direction, you are navigating by dead reckoning. Divers frequently use both pilotage and dead reckoning for underwater navigation. Natural Navigation There are many aids to navigation in the underwater environment (figure 7-4). A diver can estimate his or her approximate location reasonably well by means of natural aids to navigation. One aid is sand ripples on the bottom, which usually run parallel to the shore and are steepest and closest together nearer the shore. Other natural aids include sun and shadows, direction of water movement, orientation of certain stationary marine life (such as sea fans or kelp), bottom contour and depth, formations, and underwater landmarks (such as a wreck). Surge is stronger in shallow water near shore or around pinnacles and decreases in deeper water. Another key to natural navigation is the use of a dive pattern, which is the total course or dive path to be followed on a dive (figure 7-5). The pattern may be square, rectangular, or some other shape. The configuration of the pattern is not as important as buddies agreeing upon it prior to a dive and adhering to the general pattern during the dive. A dive pattern with square corners is easiest to perform, while a circular pattern is the most difficult. Circular patterns are not recommended unless a tethered line is used to circle its anchor point. When natural aids to navigation are used in conjunction with a dive pattern, reasonably accurate navigation is quite possible. Imagine a dive with a square dive pattern where you begin diving at the anchor of a boat, parallel to shore, and at a depth of 15 m (50 ft) and with the sun on your right. With 175 bar (2,500 psi) of air in your tank upon reaching the bottom, you and your buddy move Sun Shore 40' 1000 psi 50' 2500 psi Boat Sand Ripples 1500 psi 2000 psi Current FIGURE 7-4. USING NATURAL AIDS FOR NAVIGATION
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