[fitswcs] Units along spectral axes

[David Berry]

On Thu, 9 Jun 2005, William Thompson wrote:

> David Berry wrote:
>
>  > There was quite a bit of discussion of the handling of units on the IVOA
>  > lists a few months ago. The practical problem is that there is an awful
>  > lot of existing data out there which uses "nearly but not quite FITS"
>  > units syntax, and most software systems will need to be able to
>  > interpret these units variations. There were three basic approaches
>  > disccused on the IVOA lists:
>  > > 1) Get everyone to change all their existing data archives to
>  > correct non-standard units strings to FITS-WCS paper I syntax. Not
>  > practical!
>
> 	...
>
> I doubt that those existing archives use the "4-3" notation of Paper III to
> implement non-linear projections of the coordinates.

I agree they probably won't use the "4-3" notation, but some will use the
"4" notation for linear axes (e.g. CTYPE1  = 'WAVE    ') since these were
being used before the paper came out. You still need to be able to
interpret the axis units, even if the axis is linear.

> My main concern is making
> the projection equations work out correctly--the issue of interpreting the data
> is another matter.

Sorry - I mis-used the word data. I was referring to axis values
(including CRVAL values, etc) rather than pixel values.

David