There are many factors effecting the reliability and accuracy of the fiducial location. It is hard to say that one location operator can always precisely find the correct locations of the fiducial marks. In our approach, the two location operators' results are integrated into one as the final optimal location using the following formulas:
where and are the roundness of a fiducial obtained previously; and x, y and R are the final coordinates of the center of a target and the new roundness value. The formula is found useful in practice. We consider the fiducials are symmetrical and the roundness should be close to 1. If the roundness obtained ( or ) is 1, we just use either Wong or Föstner location operator.