4.4 Current Status

In support of this proposal, Calabretta has developed the WCSlib software package. It is available under a GNU Public License through the NRAO FITS archive described in Section 6.2.

The Greisen and Calabretta (1996) proposal is still under discussion in the FITS community and may undergo further changes. The following aspects of the proposal have wide acceptance already and should be generally understood by FITS readers:

• Index point at the center of a pixel
• Transformation matrix (future applications are expected to follow the PC notation and usage, although there are many existing files with the CD notation and combined rotation-scaling usage)
• SIN, TAN, and ARC projections, with the standard coordinates and the native coordinates locally parallel at the tangent point, as in the AIPS convention.
• The conventions for including the name of the coordinate system and projection in the value of CTYPEn.
• The coordinate keywords described in Section 4.3.

Trial implementations and discussions will continue before a standard for world coordinates becomes part of the overall FITS standard. Among the questions still under discussion are the following:

• Is there a way to use this formalism for spectra, time series, and unequally spaced data?
• Is the notation of section 4.2.1 the best choice for multiple WCS used for the same image?
• Can the pixel correction image method be implemented for complicated FITS files, for example, those with many images, each with its own correction?
• Should the parallel use of the CROTAn notation continue, and, if so, how long?
• Can agreement be reached on a more specific algorithm for choosing the CTYPEn values to be used to identify future coordinate systems and projections?
• Is the description of the common projections (e. g., SIN,TAN) now too complicated?
• Should all of the projections be included if this proposal is adopted as the standard, or should only the basic projections be included, with the rest as appendixes?

Keep up with the current discussion in the forums described in section 6. Make certain that the community has reached agreement in the areas described above before implementing any of those features in archival data sets.