1. An image forming method comprising: an image input function, to which stereo images including control points or orienting points overlapped with one another are inputted; a storage function for previously storing ground coordinate values of control points or orienting points; and a determining function for determining, based on the stereo images formed by said image input function, necessity of re-photographing and necessity of changing a photographing position, a control point or an orienting point position; wherein an ortho-image is formed from stereo images inputted by said image input function based on image coordinate values or photographic coordinate values and said ground coordinate values of the control points or the orienting points.