another compass nearby, so they can not be checked by putting them together. Dip The magnetic field of the earth is tilted from the horizontal. The angle is called dip and varies with place, so compasses can be corrected for different zones. This is a factory process. A compass made for the northern parts of the northern hemisphere will tilt badly in the southern hemisphere, in some cases to the extent that it will jam if held horizontal. Using guidelines Also known as cave lines, distance lines, penetration lines and jackstays. These are permanent or temporary lines laid by divers to mark a route, particularly in caves, wrecks and other areas where the way out from an overhead environment may not be obvious. 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. Line markers In cave (and occasionally wreck) diving, line markers are used for orientation as a visual and tactile reference on a permanent guideline. Directional markers (commonly arrows), are also known as line arrows or Dorff arrows, and point the way to an exit. Line arrows may mark the location of a "jump" location in