The invention relates to a method and an apparatus for determining a position of at least one structure element of a photolithographic mask, wherein the method comprises the following steps: (a) providing a reference image of the at least one structure element; (b) deriving a data record for the reference image, said data record comprising metadata relating to the reference image; (c) providing at least one measured image of the at least one structure element of the photolithographic mask; and (d) optimizing the reference image by use of the derived data record and correlating the at least one measured image and the optimized reference image.