Abstract: A method and apparatus for encoding and detecting data which can be represented in a physical array of modules recorded on a medium. Information is encoded in "image-blocks". An image-block comprises a plurality of "sub-blocks". Sub-blocks comprise a plurality of "modules". A module is the smallest unit of information within the image-block. Discrete contiguous portions of each sub-block of an image-block taken together are encoded into an outer codeword. A plurality of these outer codewords are generated and recorded across each sub-block. A portion of the information that is to be recorded within each sub-block is encoded in a plurality of inner error detection and correction codewords, each preferably stored entirely within one corresponding sub-block. Accordingly, small scattered random errors can be corrected locally by using information entirely contained within the sub-block.