Time transformation

[David Berry]

On Thu, 2 Apr 2020 at 15:55, CresitelloDittmar, Mark
<mdittmar at cfa.harvard.edu> wrote:
>
>
> You mean like  DATE => MJD?  GMT - PST?
> I think these are in the same level as ENERGY-FREQUENCY-WAVELENGTH, which are really standardized transforms which are basically considered different forms of the same value.  Which is why they are in the Coords model as different 'flavors' of  Time coordinate rather than having a single time coordinate and using transforms to convert.

I think it depends to what extent we want to limit the use of the
Transform model. Being able to describe the mapping between different
coordinate systems in a single physical domain  seems like a
reasonable goal for a Transform model. For instance, if I have an
image in which the WCS gives the (ra,dec) as a function of pixel
position, I may want to create a copy of that image that gives the
(l,b) of each pixel instead of (ra,dec). To do this I would need to
modify the WCS by tagging on a Mapping to convert (ra,dec) to (l,b).

Original WCS:   (pixel) -- mapping 1 -> (ra,dec)
New WCS: (pixel) -- mapping 1 -> -- mapping 2 -> (l,b )

i.e. "mapping_1" is the pixel to (ra,dec) mapping from the original
image, and "mapping 2" is the (ra,dec) -> (l,b) mapping.

Being able to modify a WCS so that it represents a different physical
coordinate system seems like a reasonable use-case. But maybe one that
can be deferred to a later date.  The beauty of the sort of system we
are creating is that it is is easy to extend it. What matters most is
that we get the right definition of mappings, transforms, operations,
axes and so on.

David