Method of evaluating flatness of a two-dimensional surface area

A method of evaluating flatness of a two-dimensional area of a surface of an unfinished workpiece prior to performance of a process step in which a prescribed operation is performed on the surface by a tool, the method comprising:

• a) at each of multiple locations within a two-dimensional coordinate system for the surface which comprises mutually perpendicular X- and Y-axes, measuring Z-axis coordinate data for elevation of the surface along a Z-axis which is perpendicular to both X- and Y-axes thereby defining X-axis, Y-axis, and Z-axis coordinate data at each location;
• b) defining a best-fit plane which best fits to the combined X-axis, Y-axis, and Z-axis coordinate data at all locations;
• c) calculating a flatness value for the surface by subtracting from the Z-axis coordinate data of the highest elevation of all locations, Z-axis coordinate data of the defined best fit plane at the location of the X-axis, Y-axis coordinate data of the location having the highest elevation of all locations to yield a positive difference, subtracting from Z-axis coordinate data of the defined best fit plane at the location of the X-axis, Y-axis coordinate data of the location having the lowest elevation of all locations, the Z-axis coordinate data of the lowest elevation of all locations to yield a negative difference, adding the absolute value of the negative difference to the positive difference to calculate a sum representing the flatness value of the surface;
• and
• d) comparing the calculated flatness value of the surface with a maximum allowable flatness value for the surface.