Call for proposals for SIAP Version 2

[Arnold Rots]

John Good wrote:
> 
> 
> Roy -
> ...
> 
> This is dead wrong.   Coordinate syntax parsing, coordinate transforms,
> and polygonal constraints (which includes the "cone search" as a single-
> constraint subset) are all straightforward and should be an integral part
> of any server-side search engine (image or catalog).  NVO should be
> promulgating knowlege on how to do this (and portable code snippets,
> where appropriate), not characterizing it as a complex issue that needs
> third-party middleware.
> 

Though I in general agree with this, the comments below are too
restrictive.  A convex polygon may be sufficient for queries, but
certainly not for general applications.  And one has to be careful
when dealing with projections.  Great circles are straight lines in
TAN projections, but not in any other.  Concave polygons and regions
with holes will need to be dealt with.

> 
> ...
> 
> I admit to a bias here.  I have yet to see a need for anything more complicated
> that a convex polygon on the sky as a region definition (i.e. no annuli or other
> 
> regions with holes).  With that constraint, I maintain that the following are
> all
> equivalent (in that a single location-filtering library can handle them all):
> 
>    o  Convex polygon in any coordinate system.  This is just
>        a set of planes cutting the sphere.  Usually great circle
>        planes but just as easily small circles (e.g. latitude lines)
> 
>    o  Point-radius "cone" searches (since this is just a single plane)
> 
>    o  Box on the sky (if you assume great circles connecting the
>        the corners of the box, this is both a four sided polygon
>        as above (you just parameterize it differently) and is exactly
>        the same as the coverage of a Gnomonic projection (TAN)
>        image of the same size.
> 
> For many real-world images, connecting corners by great circles is close
> enough for coverage checking, so I would maintain that a FITS header
> (translated to a box) can also be used as a region definition.
> 
> 
> ...
> 
> - John
> 
> 
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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 head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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