Boxes and Polygons in ADQL/STC. Questions and recommendation.

[Patrick Dowler]

On Monday 26 October 2009 06:58:51 you wrote:
> Patrick Dowler wrote:
> > Note that postgresql+pg_sphere has the same limitations (except sbox, so
> > have to use spoly directly); I also found that both STC-S and pg_sphere
> > are limited to simple polygons and do not support unions of disjoint
> > regions or holes inside polygons.
> 
> Hi Pat,
> 
> Could you expand on this?  That's not surprising for PgSphere which
> doesn't purport to do anything more than circles and polygons (and its
> boxes in a limited way), but I had understood that REGION's could be
> quite complex.  Is that not the case?

> I think I still need the first type, but from your comments it sounds
> like any REGION currently specifiable can be represented as a single
> polygon.  Is that right?

STC in general does allow for such complex regions, but the STC-S 
serialisation does not support the "operators" (add,subtract, etc). so you 
cannot describe something like several disjoint shapes as a single region.

See the last paragraph of Section 3 in

http://www.ivoa.net/Documents/Notes/STC-S/STC-S-20071205.html

So while you could have such FOV in your system (we do, with a 1-n join 
between the main table and a "PositionSamples" table with the spoly column) 
you cannot write them out as a single STC-S value in the result VOTable. That 
was what I meant.

-- 

Patrick Dowler
Tel/Tél: (250) 363-0044
Canadian Astronomy Data Centre
National Research Council Canada
5071 West Saanich Road
Victoria, BC V9E 2M7

Centre canadien de donnees astronomiques
Conseil national de recherches Canada
5071, chemin West Saanich
Victoria (C.-B.) V9E 2M7