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FITS World Coordinate System (WCS)
Primary WCS Documents
An elaborate set of FITS conventions has been defined to specify the physical, or world, coordinates to be attached to each pixel of an N-dimensional image. By world coordinates, one means coordinates that serve to locate a measurement in some multi-dimensional parameter space. A common example is to specify the Right Ascension and Declination on the sky associated with a given the pixel location in a celestial image. An archive of discussions about WCS issues that have taken place on the fitswcs@nrao.edu email exploder is available on the NRAO web site.The WCS conventions are defined in a set of formal papers. The first 3 papers have been approved by the IAU-FWG and are now also described in the FITS Standard document.
- Representations of World Coordinates in FITS (Paper I), Greisen, E. W., and Calabretta, M. R., Astronomy & Astrophysics, 395, 1061-1075, 2002. (PDF, HTML).
- Representations of celestial coordinates in FITS (Paper II), Calabretta, M. R., and Greisen, E. W., Astronomy & Astrophysics, 395, 1077-1122, 2002. (PDF, HTML).
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Representations of spectral coordinates in FITS (Paper III), Greisen, E. W., Calabretta, M. R., Valdes, F. G., and Allen, S. L.,
Astronomy & Astrophysics, 446, 747-771, 2006.
(PDF,
HTML).
- The authors' Web sites, with supplemental background WCS information: Eric Greisen, Mark Calabretta
- An unofficial errata for Papers I, II, and II: Postscript or PDF format.
- "Representations of Time Coordinates in FITS" (Paper IV), Rots, A., Bunclard, P., Calabretta, M., Allen, S., Manchester, R., Thompson, W. - This paper is currently under active development and review.
- "Representaions of distortions in FITS world coordinate systems", Calabretta, et al., latest draft (22 April 2004): Postscript, PDF,
Other WCS papers
- Mapping on the HEALPix grid, Calabretta, M. R. & Roukema, B. F. MNRAS, 381, 865, 2007 introduces the HPX spherical projection type as an addendem to the 2nd WCS paper listed above. The HEALPix projection, as defined in section 6 of this paper, was approved as part of the official FITS WCS standard by the IAU-FWG on 26 April 2006.
- Coordinate Systems for Solar Image Data, Thompson, W., Astronomy & Astrophysics, 449, 791, 2006 - a proposed set of formal systems for descibing the coordinates of solar image data, including heliographic and heiocentric coordinates.
- A proposal by Steve Allen and Doug Mink for concatenating WCS keywords.
- Conventions for Mosaic Images - a draft document by Steve Allen.that discusses the need for standardized FITS keywords for reporting of world coordinates in mosaic images.
WCS Software
The following software libraries are available to help simplify the implementation of the projections and coordinate transformations defined in the World Coordinate System conventions.- WCSLIB - Mark Calabretta's subroutines for spherical projections and coordinate transformations.
- WCSTools - Doug Mink's subroutines and utilities to convert between sky and image coordinates.
- AST library - Astrometry and World Coordinate System handling routines from the U.K. Starlink Project. Includes graphical display routines for plotting axes and labelled coordinate grids.
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