250 years, although the instruments (compass and tape) have become smaller and more accurate. Since the late 1990s, digital instruments such as distometers have started to change the process, leading to the advent of fully paperless surveying around 2007. The main variation on the normal methodology detailed below have been devices such as LIDAR and SONAR surveyors that produce a point cloud rather than a series of linked stations. Video-based surveying also exists in prototype form. Surveying A survey team begins at a fixed point (such as the cave entrance) and measures a series of consecutive line-of-sight measurements between stations. The stations are temporary fixed locations chosen chiefly for their ease of access and clear sight along the cave passage. In some cases, survey stations may be permanently marked to create a fixed reference point to which to return at a later date. The measurements taken between the stations include: direction (azimuth or bearing) taken with a compass inclination from horizontal (dip) taken with a clinometer distance measured with a low-stretch tape or laser rangefinder optionally, distance to surrounding walls – left, right, up, down (LRUD) Coincident with recording straight-line data, details of passage dimensions, shape, gradual or sudden changes in elevation, the presence or absence of still or flowing water, the location of notable features and the material on the floor are recorded, often by means of a sketch map. Drawing a line-plot Later, the cartographer analyzes the recorded data, converting them into two-dimensional measurements by way of geometrical calculations. From them he/she creates a line-plot; a scaled geometrical representation of the path through the cave. Finalising The cartographer then draws details around the line-plot, using the additional data of passage dimensions, water flow and floor/wall topography recorded at the time, to produce a completed cave survey. Cave surveys drawn on paper are often presented in two-dimensional plan and/or profile views, while computer surveys may simulate three dimensions. Although primarily designed to be functional, some cavers consider cave surveys as an art form. Hydrolevelling Hydrolevelling is an alternative to measuring depth with clinometer and tape that has a long history of use in Russia. The technique is regularly used in building construction for finding two points with the same height, as in levelling a floor. In the simplest case, a tube with both ends open is used, attached to a strip of wood, and the tube is filled with