1. A method of determining relative displacement velocity between an image sensor and a substrate, the method comprising: causing relative displacement between the image sensor and the substrate, a reference pattern being marked on the substrate, the reference pattern comprising plural crossing points at predetermined locations on the substrate, each crossing point comprising a first line portion crossing a second line portion; during the relative displacement of the image sensor and substrate generating, by the image sensor, image data representing the reference pattern; supplying the image data representing the reference pattern to a processor; detecting by the processor, in the generated image, locations corresponding to said crossing points; and determining relationships between the predetermined locations on the substrate and the detected locations in the generated image data; and producing estimates of the velocity of the relative displacement between the image sensor and the substrate using the determined relationships.