ADQL new function to compute area of intersection?

[Walter Landry]

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