A water marking system generates a water marked image frame for conveying a payload data word by combining at least one image frame with a two-dimensional water mark pattern. The water mark pattern includes plural regions each associated with one of symbols of the payload data word. Each region includes a two dimensional water mark block selected from a predetermined set of possible water mark blocks to represent the value of the payload data symbol associated with that region. A detecting data processing apparatus detects the payload data from a received water marked image frame, which has been encoded by utilizing a block match prior probability calculator operable to correlate each region of the water marked image frame with each of the water mark blocks from the set of water marked blocks) to form for each region a set of probability surfaces.