1. A method for using a computer processor to derive significant points from a point cloud containing data representing a physical surface and defined within a three dimensional coordinate system, comprising: calculating a mean z coordinate value for a plurality of points having x, y and z coordinates within said three-dimensional coordinate systems; using said computer processor to select from said point cloud data, an initial set of points having z coordinate values which deviate from said mean by at least an initial value; constructing with said computer processor a triangulated irregular network (TIN) using said initial set of points, said TIN composed of a plurality of triangles; using said computer processor to determine if there is a significant point that exists among said plurality of points contained within an x, y extent of each said triangle; updating said TIN with said computer processor to include said initial set of points and any significant points determined to exist within said plurality of triangles; and repeating said determining and said updating steps until there are no significant points found within said triangles.