A method for computing the position, orientation and focal length of a camera is presented, designed for use in applications such as the real-time overlay of graphics on a football pitch. The method uses markings on the pitch, such as arcs and lines, to compute the camera pose. A novel feature of the method is the use of multiple images to improve the acuracy of the camera position estimate. A means of automatically initialising the tracking process is also presented, which makes use of a modified form of Hough transform.