2.4. Keyword value defaults

The original FITS paper also assumed that the coordinate keywords, if present, would all be present and, therefore did not define defaults for the standard keywords. We therefore define the defaults to be

WCSAXES	NAXIS or largest i or j
CRVALi	0.0
CRPIXj	0.0
CDELTi	1.0
CTYPEi	' ' (i.e. a linear undefined axis)
CUNITi	' ' (i.e. undefined)
PCi_j	1.0 when i = j
PCi_j	0.0 when i j
CDi_j	0.0 but see Sect. 2.1.2

These defaults provide the minimal amount of information consistent with a real axis that is not fully described but will not cause zero divides. They assert that the pixel coordinate value changes as the pixel number changes and that, by default, coordinate values on the pixel axis depend upon that axis and only that axis. The reference pixel is by default just off the data array which satisfies the needs of some software systems and has the felicitous result that the coordinate value of each pixel is its pixel number if all of the keywords take their default values. With these defaults, a program may fill its coordinate arrays with usable, if uninteresting, values before reading the FITS header, rather than constructing some scheme that changes depending on the absence of keywords in the FITS header. Because default values were not defined from the beginning and appear to be a source of confusion, we recommend that FITS writers should endeavor always to write complete WCS specifications and never to depend upon defaults.