a device for deriving the contour lines for an object from stereophotographic films taken through photographic lenses comprises steroscopic viewing apparatus for comparing the films to locate the different points on the object having a selected height and for producing first position signals representing the positions of the different points in one of the films , and a correction mechanism responsive to the first position signals for producing second position signals which represent corrected positions of the points for defining a contour line corresponding to the selected height .