A distance line, penetration line or guideline 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.[1][2] Guidelines are also useful in the event of silt out.[3]
Distance lines are often wound on to a spool or a reel.[4] Then length of the distance line used is proportional to the length needed 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 will use multiple reels of lengths from 50 ft to 1000+ ft. Reels for distance lines usually have a locking mechanism to control deployment of the line and a winding handle to help keep slack line under control. 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.
The material of line used on any given distance line will vary based on intended use.[4] A common line used is 2mm / 0.08 inch polypropylene line.
The use of guideline for navigation careful attention to line following, referencing, positioning, teamwork, and communication.[3]
In 1977, Sheck Exley published Basic Cave Diving: A Blueprint for Survival which pointed out that the lack of a continuous guideline was one of five contributing factors in cave diving accidents.[1]