ADQL new function to compute area of intersection?

[Arnold Rots]

I was going to ask: doesn't it make more sense to define two functions:
1. return the intersection of two regions
2. return the area of a region
The combination of the two provides Alberto's area of the intersection
and separately they provide more functionality.
One might of course want to add:
3. return the union of two regions

Cheers,

  - Arnold

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Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Wed, Jun 7, 2017 at 1:15 PM, Walter Landry <wlandry at caltech.edu> wrote:

> alberto micol <amicol.ivoa at googlemail.com> wrote:
> > Dear All,
> > cc: Bruno, for the Standing Committee on Science Priorities <
> http://wiki.ivoa.net/twiki/bin/view/IVOA/IvoaSciencePriorities>
> >
> > The following science use case cannot be accomplished without a new ADQL
> function:
> >
> > I want to find images of the Galactic Centre in two bands (J and H)
> provided that the images
> > in the 2 bands overlap, in order to get the colours (J-H) of the sources
> imaged by both.
> > The overlapping region must be big enough so that I can detect sources
> > in both images: an overlap of, say, 0.001 square degrees would be
> irrelevant for my science case.
> >
> > I’ll show in the PS how this can be achieved using ObsCore with a
> user-defined function.
> >
> > The issue is that, to avoid images just “touching” each other, we had to
> create a user-defined function
> > that computes the area of the intersection of the two regions.
> >
> > Issues: (1) only those users that read the documentation or the
> TAPRegExt will know
> > that such functionality exists, and (2) this function cannot be used
> when querying other data centres.
> >
> > Can I suggest to introduced a new function in ADQL 2.1 which can be used
> to compute
> > the area of the intersections between two regions?
>
> It sounds like what you actually want is the distance to the edge, not
> the overlap area.  I needed something like that.  We wanted the "most
> centered" image, so we sort results by the distance to the edge.  It
> required a bit of hacking on our end.
>
> With that said, I am not a fan of new functions that can not be
> implemented via simple rewriting on GIS backends.
>
> Cheers,
> Walter Landry
>
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