Registry data structures

[Gretchen Greene]

Clive,

This article
http://research.microsoft.com/research/pubs/view.aspx?tr_id=736 is
fairly specific to the SDSS project however you might be able to extract
concepts and absorb  the summaries. 

Region intersections make use of HTM and HEALPix spatial indexes in this
example.  You might already be familiar with these so I'm not going to
go into much detail.  While they are currently coupled into missions and
projects,  they can be used at a higher level (libraries are available)
for performing region intersections.  The current VO STC definition
incorporates region specification elements which conform to the
algebraic constraints they need.

Patricio may be interested in looking into this as well as the
bitmasking since there
are libraries developed.  Not sure where to go with HEALPix,  yet HTM
http://www.skyserver.org/HTM/

Rather than a pixel/bitmap,  these indexes functionally provide rapid
geometric calculations.  If you look further,  they are highly efficient
trees for spatial mapping into 3-D space.  

A specific use case that we have experimented with using HTM is to find
intersecting HST and SDSS datasets (this was prototype work done a
couple years ago,  sparse fields vs. all-sky).  With computing the
footprint of the WFPC2 pixels (polygon spec) and,  with SDSS the larger
stripe polygon,  one could rapidly join sets.   At the time is was
fairly confined between two similar RDMS systems.  Now with web
serviceable footprint functions,  the potential is there for doing this
between remote sites (OpenSkyQuery for example,
http://openskyquery.net/Sky/skysite/Browse.aspx).  While there is still
lots to be done for fully developing the polygon intersection
capability,  it is a matter of dedicating the effort to it which up to
now has been sidelined until the standards structures are further
evolved.  I think we are rapidly approaching this capability and this is
one path that hopefully will be pursued.

Cheers, Gretchen






-----Original Message-----
From: owner-registry at eso.org [mailto:owner-registry at eso.org] On Behalf
Of Clive Page
Sent: Monday, June 28, 2004 5:24 AM
To: registry at ivoa.net
Subject: RE: Registry data structures


On Fri, 25 Jun 2004, Gretchen Greene wrote:

> We have spent a good bit of time working with the problem you mention 
> below with interoperating between sparse observations vs. large 
> surveys (HST & SDSS for example).  Not sure if you are familiar with 
> the idea of 'footprint' services (Szalay).

I've seen references to the idea, but not seen any details of how it
might be implemented.  Is there anything online that you can point to?


-- 
Clive Page
Dept of Physics & Astronomy,
University of Leicester,
Leicester, LE1 7RH,  U.K.