if the machine is moved in the direction of the arrow s it follows the track curvature while the shifting receiver follows the vertical beam fr of the laser along the chord having a length g the movement of this receiver depending on the versed sine of the arc externally of the median line thus for example if this receiver is at point a1 it has covered the distance corresponding to the versed sine or rise f1 of course in this case the relative measurement base respectively the reference line must not follow the receiver since this line determines the position of the working clamps at point b therefore the rise f1 is calculated as a function of the curve and the reference line is shifted by a distance f1 in relation to the receiver externally of the curve similarly when the laser receiver rr is at point a2 the reference line is moved to the extent corresponding to the versed sine or rise of arc f2 which is the distance between the position of receiver rr adjusted on the beam and the reference line determining the position of the working members on the desired curve fig7 further illustrates in diagrammatic cross section the track at the above defined point a2 the receiver rr has covered a distance f2 externally of the median line but on the other hand the front end of reference line lr extending at right angles to the plane of the figure must also be moved through this distance f2 to the desire position i e the centre of the track consequently the front.