A method and apparatus are provided for compressing data representing a plurality of elements of an array. This data is compressed by generating at least two sets of reduced size data from a filtered version of the original data. The elements of each set of reduced size data are representative of a plurality of elements of the original data. The original data is then used to generate modulation data, each element of the modulation data specifying how to combine the sets of reduced size data to generate an approximation to the original data. The sets of reduced size data and modulation data are stored as compressed data. Further method and apparatus are provided for accessing these compressed data and for decompressing it.