of the twenty-seven cubical grids into a table; andrepeating step (b6) until all cubical grids have been related. 9. The method as described in claim 8, wherein the step (d) comprises steps of:(d1) obtaining a serial number of the cubical grid which confines the selected point, and searching twenty-six cubical grids which are adjacent to the cubical grid according to the serial number;(d2) judging whether quantity of the points confined in the searched cubical grids is more than a predetermined value;(d3) searching other cubical grids which are adjacent to one of the searched cubical grids until the quantity of the points is more than the predetermined value; and(d4) computing a distance between the selected point and each of the points that is confined in the searched cubical grids to obtain the near points of the selected point according to the distances. 10. The method as described in claim 7, wherein the step (e) comprises steps of:(e1) respectively arranging x-coordinate values, y-coordinate values and z-coordinate values of the selected point and the near points into three arrays in an ascending sequence or in a descending sequence;(e2) determining whether at least one of the x-coordinate value, y-coordinate value and z-coordinate value of the selected point is at the ends of the corresponding array; and(e3) ascertaining the selected point is a noise point if at least one is at the end of the corresponding array; or(e4) ascertaining the selected point is a normal point if none of the x-coordinate value, y-coordinate value and