# [QUANTITY] Data Model for Quantity v0.5

Brian Thomas brian.thomas at gsfc.nasa.gov
Wed May 12 08:49:57 PDT 2004

```On Wednesday 12 May 2004 11:41 am, David Berry wrote:
> Brian,
>
> > The whole point of moving getCurrent[Coords|Value]Quantity to the locator
> > is that you  may have more than one locator open for a Q at a time. If
> > you place it on the parent Q, then you are restricted to traversal only
> > in one frame at a time which is a needless restriction. After all, there
> > should be no "favored frames", it is the job of the locator (I think) to
> > pick which frame it wants to use for traversal (and to instruct the
> > parentQ which values to return).
>
> This was another issue on which we retired gasping to our corners. The
> concept of traversal "in a Frame" means nothing to me, and consequently I
> can see no need for locators. For instance, you have an image of the south
> celestial pole in which "lines" of constant dec are actually circles -
> in this situation what does it mean to "traverse the image in (RA,Dec)"?
> What order are the pixels returned in? As far as I can see it only makes
> sense to return them in the natural order they are stored in the image
> (1,1), (1,2), (1,3), ... (2,1), (2,2), (2,3), ...

You are (perhaps again) only thinking of your favorite example and
not the larger picture here. SkyVectors are a single axis for the
purposes of holding data in the Q, and therefore, there is no way
to reverse the order of traversal.

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), ...

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]

```