Abstract: A method and a device for determining the desired middle line of a cylindrical object, such as a log. For example, to determine the rotating middle line of an object to be rotary-cut, the invention provides a method in which the diameter of the object and its change is recorded with at least one camera or some other image producing device from several angles, and the images thus produced are stored in a computer memory, after which the desired middle line can be calculated using ordinary logic and the image most suitable for the purpose.