Abstract: An elongated workpiece is treated, e.g. rolled, in a production system where treatment devices move the workpiece longitudinally in a horizontal travel direction and at a generally constant travel speed along a horizontal longitudinal path and a control computer is connected to the treatment devices. Straightness of the workpiece is measured by gripping the workpiece at a pair of longitudinally spaced supports, applying tension to a longitudinally extending horizontal portion of the workpiece between the supports, and continuously moving the workpiece in the travel direction at the travel speed with a lower surface of the portion substantially unsupported. At least periodically a vertical spacing is measured between the portion and a straight longitudinal reference line extending in the direction between the supports parallel to the workpiece at the supports. The vertical-spacing measurement are fed to the computer for use controlling the devices.