[fitswcs] WCS documents

[Doug Tody]

Stephane - What you describe below is what we called the axis map,
used to map the image axes to WCS axes (it is mentioned at the end
of section III).  We suggested that this could be skipped for FITS
WCS as FITS is generally optimized for static archival data and
the WCS axes can be reordered if dimensional reduction is necessary.
But as you say, it would avoid the need to do so in many cases.
It is especially useful when a WCS has to be maintained in a runtime
context where images are being modified or created.

	Doug


On Wed, 26 Sep 2001, Stephane Guilloteau wrote:

> I have read Doug Tody proposal, and find quite a number of convincing
> arguments
> to decouple the WCS from the NAXIS problem.
> 
> However, as far as I understood, the ordering of the WCS axes must remain
> the same as the ordering of the
> Image axes (NAXISi). I think this is an unnecessary restriction which may
> lead to cumbersome
> code, and that we should look into the alternative, which is to specify a
> mapping between the
> WCS axes and the image axes, e.g. by a WCS keyword like
> 
>     WCSAXISi    = j
> 
> indicating that image axis J corresponds to WCS axis I
> 
> or vice-versa (which is even more convenient)
>     WCSj          = i
> indicating that the image axis J must take its WCS coordinate from WCS axis
> I
> 
> The latter form allows to carry on very easily all the World Coordinates
> whatever transposition is done on the data.
> It also make things somewhat easier when a "dumb" image must be associated
> to a WCS space with no  associated data as in Doug's example. Simple copy of
> keywords would do the job. Keyword inheritance could be used to derive WCS
> coordinates in IMAGE XTENSION, even if the image extensions represent
> different things like a sky map first,
> than a long slit spectrum.
> 
>         Stephane
> 
> 
> Dr. Stephane GUILLOTEAU
> ALMA European Project Scientist                Phone: (33) 476 82 49 43
> (IRAM)
> IRAM                                                         FAX:     (33)
> 476 51 59 38  (IRAM)
> 300 Rue de la Piscine
> F-38406 Saint Martin d'Heres
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> guillote at iram.fr    (IRAM)
> 
> 
>