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FITS World Coordinate System (WCS)
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 currently described in a set of 4 papers. The first 3 papers have been approved by the IAU-FWG and are now officially recognized as part of the FITS Standard.
Other WCS papers:
- Representations of World Coordinates in FITS, Greisen, E. W., and Calabretta, M. R., Astronomy & Astrophysics, 395, 1061-1075, 2002. (PDF, HTML). (These links to the publisher's web site are currently available by subscription only). Reprints are available from the author's web site in PDF format.
- Representations of celestial coordinates in FITS, Calabretta, M. R., and Greisen, E. W., Astronomy & Astrophysics, 395, 1077-1122, 2002. (PDF, HTML). (These links to the publisher's web site are currently available by subscription only). Reprints are available from the author's web site in PDF format.
An unofficial errata for Papers I and II is available in postscript or pdf format.
- Representations of spectral coordinates in FITS, Greisen, E. W., Calabretta, M. R., Valdes, F. G., and Allen, S. L., Astronomy & Astrophysics, 446, 747-771, 2006. (PDF, HTML). (These links to the publisher's web site are currently available by subscription only). Reprints are available from the author's web site.
- "Representaions of distortions in FITS world coordinate systems" by Calabretta, et al. The latest draft of this paper are available from the author's web site. A related proposal by Steve Allen and Doug Mink for concatenating WCS keywords eventually may be incorporated into the final version of 4th WCS paper.
- 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 (PDF or PS format) by Bill Thompson proposes a set of formal systems for descibing the coordinates of solar image data, including heliographic and heiocentric coordinates.
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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Last revised: Thursday, 15-Nov-2007 14:25:22 EST
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