1. A method of digitising a surface of a workpiece comprising the steps of: scanning the surface with a probe to produce a digitised coordinate data signal representing coordinates of a plurality of points on the workpiece surface; and filtering said coordinate data signal to reject portions of said coordinate data signal representing some of said points, wherein the filtering step comprises the steps of: selecting portions of said coordinate data signal representing first, second and third points, produced in that order, on the surface, creating a vector extending from the first point through the third point, determining whether said vector lies within a predetermined tolerance distance from the second point, and removing the portion representing the second point from said coordinate data signal to reject the second point if said vector lies within said tolerance distance.