Specialist applications

Applications can be as sophisticated and specialised as they like in their use of data quality, and are at liberty to assign specific meanings to values of data quality, e.g. a fiducial mark, vignetting, saturation. The details of how data-quality information is encoded within the 8 bits are specific to each kind of data source and specialist package. A description of how quality will be interpreted must be given in the documentation for each package that uses the technique. However, it is possible to identify some general features of data-quality processing.

Each data-quality value can be regarded as a set of bit groups, each containing one or more bits. The recommended approach is to use single bits, each with an independent meaning, to form eight 1-bit deep logical masks. However, it is also permissible to take several bits (which ought to be contiguous) and interpret them as a positive integer. Single bit fields are used to contain a flag (1 = .TRUE., 0 = .FALSE.) for some feature (e.g. ``pixel in fiducial''). Multiple-bit fields are used to contain code numbers or degree of quality.

It is envisaged that most manipulation of data-quality values will be done quite transparently by those applications which know how to use them to advantage, without the user being aware of the mechanism. However, it is expected that there will be some cases where users will want to manipulate data quality explicitly, and there will be various data-quality editing applications, often using graphics or image displays. For example, there will be instances where the user wishes to view a picture on a display and select which pixels are to be temporarily flagged as ``wrong'', rather than trust some automatic algorithm.

Since the data quality codes are stored separately from the actual data, data-quality editing will normally be a reversible process, leaving the data values themselves untouched.

(n.b. The implementation of data quality is largely unchanged from the Wright-Giddings proposal.)