Quality

To flag a data value as ``bad'', an associated data-quality value can be used. This is an array of 8-bit positive integers, one per element of the data array with which it is associated (a single value, applying to all elements of the data array, is also possible, but this will rarely be useful), whose bits describe, in various ways, attributes of the data value concerned. The recommended way to use data quality is to regard the 8 bits as eight independent logical masks, one mask per attribute.

As its name implies, data-quality is a qualitative description of the data value. It is frequently used to flag bad pixels, but is also useful for ``good'' attributes, e.g. which regions of a picture constitute the sky sample. It is not in any sense an error estimate (though groups of bits might be used to convey some numerical meaning); it finds application in circumstances where an error estimate is not meaningful. Here are some examples of how data quality might be used:

• In an image where the intensities of some pixels have been digitally truncated, there is only a lower limit to the actual incident intensity; the upper limit is unbounded. Data-quality could be used to flag this condition, and application programs could then decide whether to use the pixel value or to treat it as missing.
• Data quality is useful where a pixel has an accurate intensity, but has to be interpreted in a different way from other pixels. The case of pixels affected by fiducial marks (e.g. reseaux) is a common example of this. For most parts of the processing, such pixels must be excluded. However, in an application which locates the fiducial marks themselves, they would clearly be crucial.
• Where parts of a picture are vignetted, data-quality allows these regions to be ignored when appropriate (at the discretion of the user, for example) without losing what information they contain.

Sometimes a simple true/false mask is not enough. In such cases it is possible to use combinations of bits to indicate both the presence of the condition and to what subclass of that condition the pixel belongs. For example, a group of three data quality bits could be used not only to flag saturation but also to grade the degree of saturation, on a scale of 1-7.

Clearly, not all values stored in the data system will have associated data-quality; that would be unnecessary and quite wasteful of resources. Normally, data-quality values are associated with basic observational or measured data.