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5.3. Tables

Binary extension tables (Cotton et al. [1995]) use the NAXIS2 keyword, invented for simple images, to specify the number of rows in a table. It has been suggested that, if the rows of the table are regularly spaced in some world coordinate, that world coordinate could be described with the other axis-2 keywords such as CTYPE2a, CRVAL2a, CDELT2a, and PC2_2a. Since we know of no software system using this convention with these general keywords, we deprecate the suggestion. There are very powerful and useful general operators such as sorting, editing, and concatenation which alter the value of the row number and therefore corrupt the value of the implied coordinate. If FITS were solely used as an interchange mechanism, these operators would not be relevant. But FITS is now used as the internal format of several software systems for which the general operators are important. Initial row number can be recorded as a column in tables and associated with a physical coordinate via keywords described in Sect. 3.