a cave when two are placed adjacent to each other. Two adjacent arrows facing away from each other, mark a point in the cave where the diver is equidistant from two exits. Arrow direction can be identified by feel in low visibility. Non-directional markers ("cookies") are purely personal markers that mark specific spots, or the direction of one's chosen exit at line intersections where there are options. Their shape does not provide a tactile indication of direction as this could cause confusion in low visibility. One important reason to be adequately trained before cave diving is that incorrect marking can confuse and fatally endanger not only oneself, but also other divers. Using surface control personnel In some circumstances divers may be directed by their surface control personnel. This requires a method of communication between the surface team and the diver. Both voice communications and line signals may be used to direct the movement of the diver and to provide other information. Surface direction may be used in scuba diving when diving under ice or conducting an underwater search, and in surface supplied diving for both these purposes and at any other time that it is useful or convenient for the dive controller to direct the movement of the diver. Surface direction is most useful when the surface personnel have a better idea of where the diver is relative to where he or she needs to be than the diver has, which can happen when the visibility is poor, or when the diver is following a search pattern controlled by the surface controller. Surface navigation for diving Surface applications for compass navigation include marking a position and finding the position using compass bearings. At least two position lines are required to fix a position, as only direction can be found using a compass. When two bearings are used a large angle between the bearings will minimize error. The angle should preferably be between 60 and 120 degrees, and near 90 degrees would be ideal. Three bearings are better as they will also give an indication of probable accuracy when plotted on a chart. The "cocked hat" or triangle where the lines intersect, shows the probable location of the position measured, and a small triangle indicates a small probable error. The angle between the three bearings should preferably be in the order of 60 or 120 degrees where available landmarks allow. In