A reference space includes a plurality of reference graphics. An image space is obtained by imaging the reference space. Affine transformation series are determined from the reference and image spaces for transcribing graphic cores of imaged graphics on graphic cores of reference graphics. If an affine-transformed positional error falls outside a tolerance, the reference space is divided into sub-spaces. Affine transformations and space divisions are repeated for out-of-tolerance sub-spaces until their estimations of distortion finally fall inside the tolerance.