Abstract: The present invention enables compression of digitized pictorial data to be achieved by comparing a data block of a predetermined size with a group of patterns relevant to that size held in a library. If a match is found the data block is represented by a code identifying the pattern on an output signal by a code identifying the pattern. If no match is found, the data block is sub-divided into smaller blocks which are compared with another group of patterns relating to the smaller block size. This sub-division process continues until the data is matched or a predetermined minimum size of data block is reached, in which case the data itself is applied to the output signal. The compressed data may be reconstituted and displayed or used to monitor the progress of a vehicle, for example.