Published in Astronomy & Astrophysics 395, 1059, 2002.

astro-ph/0207407.

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# REPRESENTATIONS OF WORLD COORDINATES IN FITS

### E. W. Greisen ^{(1)} - M. R. Calabretta ^{(2)}

^{(1)}
National Radio Astronomy
Observatory, P. O. Box O,
Socorro, NM, USA 87801-0387

^{(2)} Australia Telescope National Facility, P. O. Box 76,
Epping, NSW 1710 Australia

**Abstract.** The initial descriptions of the FITS format provided a
simplified
method for describing the physical coordinate values of the image
pixels, but deliberately did not specify any of the detailed
conventions required to convey the complexities of actual image
coordinates. Building on conventions in wide use within astronomy,
this paper proposes general extensions to the original methods for
describing the world coordinates of FITS data. In subsequent
papers, we apply these general conventions to the methods by which
spherical coordinates may be projected onto a two-dimensional plane
and to frequency/wavelength/velocity coordinates.
Methods: data analysis - Techniques: image processing -
Astronomical data bases: miscellaneous

**Table of Contents**

- INTRODUCTION

- BASIC CONCEPTS
- Coordinate definition and computation
- Basic formalism
- Linear transformation matrix
- Usage comments
- Additional points

- Coordinate dimensionality
- Keyword value units
- Keyword value defaults
- Alternate axis descriptions
- Uncertainties in the coordinates

- ALTERNATE FITS IMAGE REPRESENTATIONS: PIXEL LIST
AND VECTOR COLUMN ELEMENTS
- Multi-dimensional vector in a binary table
- Tabulated list of pixels
- Keyword naming convention
- Multiple images and the "Greenbank
Convention"
- Coordinate system cross-references

- SPECIFICATION OF UNITS

- ADDITIONAL MATTERS
- Image display conventions
- Units in comment fields
- Tables
- Conventional coordinate types

- HEADER CONSTRUCTION EXAMPLE

- SUMMARY

- REFERENCES