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 image data, locations corresponding to the crossing points; determining relationships between the predetermined locations on the substrate and the detected locations in the image data; selecting first and second equal scan-time lines in the image data, each equal scan-time line including a plurality of points; computing a plurality of points on the substrate that correspond to the plurality of points in each equal scan-time line based on the relationships; calculating first and second sets of values to convert the plurality of points in the first and second equal scan-time lines respectively to the corresponding pluralities of points on the substrate; and producing an estimate of the velocity of the relative displacement between the image sensor and the substrate and an estimate of a rotational velocity of the substrate based on differences between the first set of values and the second set of values.