A reference image updating method and apparatus used in an image-correlation system which updates a reference image when predetermined control parameters are met. An image corresponding to a displacement of a surface is captured and stored. A reference image and the captured image are compared at different offsets in a displacement direction. When the displacement falls within set predetermined displacement values or a predetermined value for a sample time that corresponds to the displacement, then the reference image is updated. By updated the reference image in this manner, systematic errors are prevented from accumulating thereby significantly removing systematic errors in the image-correlation system.