size parameters, calculated from the cave survey, length and depth. They are closely related to the definition of a cave as a hollow underground formation, large enough for human exploration, and are used to compare and classify caves, for instance in the List of longest caves and in the List of deepest caves. Whereas the cave depth is well defined as the difference between the highest point and the lowest point of the cave, its length, usually given as the sum of distances between the survey stations on the cave floor, involves considerable arbitrariness. In cave science it was long known that the cave length does not represent the cave size properly as caves are 3D objects with volume as their main and most noticeable feature, yet volume was less used because of the lack of a suitable measurement method. In 1972, at the 6th Yugoslavian congress of speleology, Jakopin proposed volume as the main cave parameter in the paper O numeričnem vrednotenju kraških objektov ("Numerical Valuation of Objects on Karst"), measured from a computer-based cave model. In 1974 he made a 3D model for approximation of cave space, based on a series of connected polygonal cross sections and used it to make a 3D survey of Skednena jama cave, a fossil ponor at the northern rim of the Planinsko polje karst field in Slovenia. By 1979 he developed a computer program to support the 3D model, to compute its vertex coordinates and to calculate the model (and cave) parameters: length, surface and volume. In 1981 Jakopin published the results for Skednena jama cave in the paper Macrostereological Evaluation of Cave Space. The model had 305 vertices, 51 cross sections, total length was 205 m, surface area 8900 m2 and volume 6,500 m3 with error estimated at below 5%. Early in 1981 a larger cave, Mačkovica, located in the same area as Skednena jama cave, was surveyed. Here the model consisted of 106 cross sections and 709 vertices for a length of 650 m and volume of 38,800 m3. Volume error was estimated at below 2%. After microcomputers became widely available in the early 1980s, Jakopin developed a different method of volume calculation which could be performed with much less computing power. Instead of computing cave model segment parameters by cutting it iteratively into ever thinner parallel slices it was based on breaking up every segment into a series of