1. A method of manufacturing a photomask, the method comprising: forming a mask pattern including a critical mask feature on a photomask; and scanning or imaging the photomask to optically retrieve, from the photomask, shape information descriptive for an outline of the critical mask feature on the photomask, wherein the shape information contains position information identifying the positions of isolated landmarks on the outline relative to each other and wherein first ones of the landmarks indicate at least one of local curvature extrema, points of inflexion, sharp bends in the curvature and local curvature-change maxima in the outline of the critical mask feature, respectively.