a computer system reviews and modifies horizons tentatively defined by a network of pairs of turnings . the curvature for each of the paths of turnings of the network in both the x and y directions is assessed . if the curvature exceeds a predetermined value in any of the paths , that path is terminated at the point of excessive curvature . also , each pair of turnings is tested to determine whether it is in a closed loop made up of four turnings which in turn make up four pairs of turnings to determine that there is three - dimensional continuity . the horizons are thereby modified if modification is required .