The position of an edge of a marker structure in an image of the marker structure is determined with subpixel accuracy. A discrete intensity profile of the edge, having profile pixels, is derived from the image pixels, and a continuous profile function of the edge is determined based on the profile pixels. Profile pixels whose intensity values are near an intensity threshold value are selected as evaluation pixels. Based on the evaluation pixels, a curve of continuous intensity is calculated. A position coordinate at which the intensity value of the continuous intensity curve matches the threshold value is selected as a first position coordinate, and the distance is determined between the first position coordinate and the position coordinate of the evaluation pixel that, from among the evaluation pixels previously selected, has the closest intensity value to the threshold value. The determined distance is compared to a predetermined threshold, and if the distance is greater than the threshold, a shift is effected, and the process iteratively performs the steps of selects the adjacent profile pixels, calculates the curve of continuous intensity, and so forth. If the distance is not greater than the threshold, the position of the edge in the captured image is determined with subpixel accuracy from all the distances determined in step g).