A system for measuring a pattern on a sample, including: a data processing system that processes a set of two-dimensional distribution data of intensities from the sample, to calculate: a set of edge points indicative of position of edges of the pattern in a two-dimensional plane from the two-dimensional distribution data; an approximation edge indicative of the edge of the pattern; an edge fluctuation data by calculating a difference between the set of edge points and the approximation edge; and a correlation between a first portion of the edge fluctuation data and a second portion of the edge fluctuation data.