# [QUANTITY] Data Model for Quantity v0.5

David Berry dsb at ast.man.ac.uk
Wed May 12 09:15:11 PDT 2004

```Brian,

> But for n-dimensional data cubes in general, say something that
> holds info on R, T, D (Radius, Temp, Density) you have 3 orthogonal
> axes, and  perhaps you wish, for purposes convenient to your application/code
> with a different ordering.. say (to use your example)
>
> (1,1), (2,1), (3,1), ...(3,3)
>
> instead of the stored order of
>
> (1,1), (1,2), (1,3), ... (2,1), (2,2), (2,3), ...

OK. I can see that people may want to step through the N-d array using
some other axis order that the usual one for their language. But this only
relates to actual array indices - how can it relate to any other
generalised non-linear world coordinate system? IN your example you say
the 3 axes are orthogonal, but what if they are not orthogonal? What if
there is some complex non-linear dependancy betwen them?

If locators are only of use in the very specific case of WCS Frames
containing axes which are orthogonal to each other, and in which there is
an exact one-to-one connection between WCS axes and pixel axes  (i.e. each
WCS axis depends only on one pixel axis), then locators should not be
included as part of the model because we are aiming at a *general* model
and we should not be forced to shoe-horn the general case intothe mould of
one very specific individual case.

Our aim should be to model the most general situation in which a WCS
Frame spanned by N axes is used to describe position in an M-dimensional
"pixel" array (note, N and M need not be equal), and in which each of the
N WCS axes depends in some non-linear way on any combination of the M
pixel axes. How do locators help in this general case?

My proposal is to to allow the selection of a "current" WCS Frame (as is
already allowed in the interface), and then for application code to
specify explicitly the coordinates (within the current Frame) in which it
is interested. What does this scheme miss? Why do we need locators?

David

> 	Why cause the application to have to create a new quantity, with the
> 	axes order reversed, when you can simply specify to the locator that
> 	you want the iteration order to change.
>
> >
> > How can you define any other order? And if you always get the pixel back
> > in the same order then what is the point of locators?
> >
> > You may say "I have already answered that" - if so, sorry, but I didn't
>
> 	Yes, its true. Answer appears above.
>
> 	=b.t.
>
> >
> >
> > David
> >
> > ----------------------------------------------------------------------
> > Dr David S. Berry    (dsb at ast.man.ac.uk)
> >
> > STARLINK project		 |	Centre for Astrophysics
> > (http://www.starlink.ac.uk/)	 |	University of Central Lancashire
> > Rutherford Appleton Laboratory	 |	PRESTON
> > DIDCOT				 |	United Kingdom
> > United Kingdom			 |	PR1 2HE
> > OX11 0QX                                Tel. 01772 893733
> >                                              01257 273192
>
> --
>
>   * Dr. Brian Thomas
>
>   * Dept of Astronomy/University of Maryland-College Park
>   * Code 630.1/Goddard Space Flight Center-NASA
>
>   *   fax: (301) 286-1775
>   * phone: (301) 286-6128 [GSFC]
>            (301) 405-2312 [UMD]
>
>
>

----------------------------------------------------------------------
Dr David S. Berry    (dsb at ast.man.ac.uk)

STARLINK project		 |	Centre for Astrophysics
(http://www.starlink.ac.uk/)	 |	University of Central Lancashire
Rutherford Appleton Laboratory	 |	PRESTON
DIDCOT				 |	United Kingdom
United Kingdom			 |	PR1 2HE
OX11 0QX                                Tel. 01772 893733
01257 273192

```