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A distance line, penetration line or guide line is an item of diving equipment used by scuba divers as a means of returning to a safe starting point in conditions of low visibility, water currents or where pilotage is difficult. They are often used in cave diving and wreck diving where the diver must return to open water after a penetration when it may be difficult to discern the return route. Guidelines are also useful in the event of silt out. Distance lines are wound on to a spool or a reel. The length of the distance line used is dependent on the plan for the dive. An open water diver using the distance line only for a surface marker buoy may only need 50 metres / 165 feet, whereas a cave diver may use multiple reels of lengths from 50 ft (15 m) to 1000+ ft (300 m). Reels for distance lines may have a locking mechanism, ratchet or adjustable drag to control deployment of the line and a winding handle to help keep slack line under control and rewind line. Lines are used in open water to deploy surface marker buoys and decompression buoys and link the buoy on the surface to the submerged diver, or may be used to allow easy return navigation to a point such as a shotline or boat anchor. The material used for any given distance line will vary based on intended use, nylon being the material of choice for cave diving.〔 A common line used is 2 mm (0.08 inch) polypropylene line when it does not matter if the line is buoyant. The use of guideline for navigation requires careful attention to laying and securing the line, line following, marking, referencing, positioning, teamwork, and communication.〔 ==Applications== Cave diving: Guide lines are used in cave diving as standard procedure whenever it is possible that the divers may be confused about the way out. In effect, this is in any dive where there is no free surface overhead and where daylight is not visible at all points of the dive. By the most common definition of cave diving, this would mean guide lines should be used for all cave dives. Wreck diving: The use of guidelines in wreck diving is very similar to their use in cave diving. Diving in low visibility: A guideline may be used for dives where poor visibility may make it difficult to return to the starting point, and there is a safety or operational requirement to return to that point. Other dives where it is necessary or highly desirable to return to a starting point: This may include dives where a shot line or anchor line is to be used for ascent, and other dives where a specific exit point is chosen for safety or convenience. Working divers may use a guide line to allow confident and efficient movement to, from and around the workplace. This may also be referred to as a jackstay. In these applications the guideline is generally more for efficiency than for safety, but it may also help the divers stay away from potentially hazardous areas. Guidelines may also be used as a means of directing tourists around a dive site, between points of interest which may be difficult to find without the line. This form of guideline may be permanently placed. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「distance line」の詳細全文を読む スポンサード リンク
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