1. A computer-implemented method for characterizing data points in a two-dimensional space, comprising: selecting a sequence of focal points from the data points and for each focal point calculating a local tangent value by: selecting for the focal point a neighborhood of data points including the focal point, selecting multiple pairs of points from among the points in the neighborhood and for each pair of points, calculating a component tangent value defined by the slope of a line defined by the pair of points, thereby calculating multiple component tangent values for the focal point, estimating a local tangent value at the focal point based on the component tangents calculated for the neighborhood of the focal point.