Abstract: A method for determining from optical measurement a measure of the misalignment, i.e. the deviation in the horizontal plane between the actual and the desired position of a rotatable elongated part (9) of a machine (1). Use is made of an imaginary vertical reference plane (.SIGMA.) lying at a distance from the machine (1). The reference plane (.SIGMA.) is determined by two base points (3, 4) which set up an imaginary base line (.GAMMA.) at an angle (.uparw.) relative to the longitudinal axis (L) of the machine (1), and by an optically reflecting device (5) disposed such that any cross-section thereof with a horizontal plane is a straight line at right angles to the imaginary reference plane (.SIGMA.). With an optical or electro-optical measuring instrument (7) equipped for angle measurement, and a further optically reflecting device (10) being placed on said machine part (9), the angle (.phi.) between the line of sight (Z) of the measuring instrument (7) and an arbitrary imaginary line (.OMEGA.