a matter of training, practice and familiarity with the use of underwater compasses, combined with various techniques for reckoning distance underwater, including kick cycles (one complete upward and downward sweep of a kick), time, air consumption and occasionally by actual measurement. Kick cycles depend on the diver's finning technique and equipment, but are generally more reliable than time, which is critically dependent on speed, or air consumption, which is critically dependent on depth, work rate, diver fitness, and equipment drag. Techniques for direct measurement also vary, from the use of calibrated distance lines or surveyor's tape measures, to a mechanism like an impeller log, to pacing off the distance along the bottom with the arms. Many skilled underwater navigators use techniques from both of these categories in a seamless combination, using the compass to navigate between landmarks over longer distances and in poor visibility, while making use of the generic oceanographic indicators to help stay on course and as a check that there is no mistake with the bearing, and then recognising landmarks and using them with the remembered topography of a familiar site to confirm position. Using natural features Landmarks Recognisable topographical features may be remembered or noted and used identify position and direction. This is particularly useful if the visibility is sufficient to see the next landmark on the route before leaving the last. Landmarks are ordinarily considered permanent or semi-permanent features, such as ridges, boulders, wrecks or clumps of weed, but use can also be made of temporary marks such as anchor cables, shot lines, jackstays and guide lines. Depth and depth variation The slope of the bottom is often a reliable indicator of the direction toward the shore, particularly when the bottom is of soft or loose material, and is not broken up greatly by rocky outcrops. This information can be checked for reliability on a sufficiently detailed chart of the area. Contours of depth running roughly parallel to the coastline indicate a slope dipping directly away from the shore, and can be used to maintain a sense of distance and orientation relative to the shore. In some places where the bottom is composed of predominantly rocky outcrops the slope may be in any direction and is not a reliable indicator of direction. Angle of the sun, changes in light level If circumstances of depth and water clarity allow the position of the sun to produce