The changes to FITS-header keywords are summarized in Table 8. As described in Paper II, for one purpose, CROTAi may in some cases be used instead of the new keywords so that the coordinate information may be understood by software systems which have yet to be converted to these new conventions.
Keyword | Type | Sect. | Use | Status | Comments |
WCSAXESa | integer | 2.2 | WCS dimensionality | new | allows WCS specification for degenerate axes, to be explicit rather than implicit. |
CRVALia | floating | 2.1.1 | value at reference point | extended | meaning of reference point forced by coord. type. |
CRPIXja | floating | 2.1.1 | pixel of reference point | extended | meaning of reference point forced by coord. type. |
CDELTia | floating | 2.1.1 | increment at ref. point | retained | meaning of increment clarified. |
CROTAi | floating | 1 | rotation at ref. point | deprecated | replaced by PCi_j and CDi_j. |
CTYPEia | character | 2.1.1 | coord./algorithm type | extended | Non-linear types have "4-3" form: characters 1-4 specify the coordinate type, character 5 is "-", and characters 6-8 specify an algorithm code for computing the world coordinate value; case dependent. |
CUNITia | character | 2.3 | units of coord. values | new | case dependent, allowed values and combinations are described in Sect. 4. |
PCi_ja | floating | 2.1.2 | transformation matrix | new | linear conversion of pixel number to pixels along coordinate axes; default = 0 (i j), = 1 (i = j). |
CDi_ja | floating | 2.1.2 | transformation matrix | new | linear conversion of pixel number to relative coordinates; default all 0 if any given, else PCi_j applies. |
PVi_ma | floating | 2.1.4 | parameter m | new | parameters required in some coordinate types; defaults are algorithm-specific; see Papers II and III for usage examples and default conventions |
PSi_ma | character | 2.1.4 | parameter m | new | parameters required in some coordinate types; defaults are algorithm-specific; see Paper III for usage example and default conventions |
WCSNAMEa | character | 2.5 | coord. version name | new | optional documentation |
CRDERia | floating | 2.6 | random error | new | uncertainty in coordinate due to random errors; default = 0 |
CSYERia | floating | 2.6 | systematic error | new | uncertainty in coordinate due to systematic errors; default = 0 |
where j is a pixel axis number 1 through 99, i is an
intermediate world coordinate axis number 1 through 99, m is a
parameter number 0
2 through 99, and a is a coordinate
description version character blank and A through Z.
PC i_ja and CD i_ja may
not occur in the same header data unit.