SuperCOSMOS mapping data constitutes a very large dataset. When held in the form of two bytes per pixel, a measurement of a full UKST plate occupies about 2Gbytes (350mm square at 10micron pixels is 1.2Gpixels). The machine measures in lanes, typically 1600 pixels wide, so the dataset comes from the machine structured as about 22 separate lanes.
The image analyser is organised to handle the data one lane at a time, and it is also convenient for astronomers wishing to take data to other sites to be free from the restriction of having to have a whole dataset on disk for processing at one time. As a result, it is advantageous to retain the lane structure in the stored data. The most convenient way to do this is to use a directory on disk to hold the dataset. The directory contains a header file, giving simple details of the layout of the data, and a series of data files, each containing the pixel values for a single lane. The tape archive (for storage or export to users) is a "tar" archive of the directory. This makes it straightforward to restore from tape individual lanes of data.