A tip-scanning atomic force microscope (AFM) can be used as a highly accurate height-measuring instrument for large samples, such as liquid crystal displays. To accurately measure the flatness or surface roughness of large samples, the xy-scanner-induced out-ol-plane motion must be known to discriminate scanner artifacts from the measured AFM images. As the topographic signals of AFM measurements contain the hysteresis of the z-scanner piezoelectric actuators, actual movements of the z-scanner were measured using a z-axis sensor glued to the actuator. The actual out-ol-plane motion of the xy-scanner was found to be less than 1 nm for a $50-{mu}m$ scan.