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<div class="moz-cite-prefix">Going this way, why not taking into
account elliptical errors:<br>
<br>
xmatch(ra1, dec1, a1, b2, pa1, ra2, dec2, a2, b2, pa2, thresold on
the Mahalanobis distance)<br>
<br>
with:<br>
- a: semi major axis<br>
- b: semi minor axis<br>
- pa: position angle<br>
<br>
returning the Mahalanobis distance, the weighted mean position and
the associated elliptical error to be able to coherently chain
cross-matches...<br>
<br>
Cheers,<br>
<br>
fx<br>
<br>
On 02/09/2016 06:01 PM, Arnold Rots wrote:<br>
</div>
<blockquote
cite="mid:CAJXToE_1QkG+65UpKq+QHTz3pTGr7bpxvANs_woov4M0RBbj8A@mail.gmail.com"
type="cite">
<div dir="ltr">
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<div>
<div>
<div>
<div>Alowing the two radii to be specified separately
would make it more<br>
</div>
explicit that they are associated with specific catalogs
and allows<br>
</div>
users to keep track of those specific catalog-dependent
values, particularly<br>
</div>
if they do cross-matching involving multiple catalogs.<br>
<br>
</div>
One might even consider returning the overlap ratio (area of
intersection<br>
over area of smallest circle) as a (admittedly somewhat bogus)
match probability .<br>
<br>
</div>
<div>Cheers,<br>
<br>
</div>
<div> - Arnold<br>
</div>
</div>
<div class="gmail_extra"><br clear="all">
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<div class="gmail_signature">
<div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>
Arnold H. Rots
Chandra X-ray Science Center<br>
Smithsonian Astrophysical Observatory
tel: +1 617 496 7701<br>
60 Garden Street, MS 67
fax: +1 617 495 7356<br>
Cambridge, MA 02138
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href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>
USA <a
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target="_blank"><a class="moz-txt-link-freetext" href="http://hea-www.harvard.edu/~arots/">http://hea-www.harvard.edu/~arots/</a></a><br>
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<br>
<div class="gmail_quote">On Tue, Feb 9, 2016 at 11:50 AM, Tom
McGlynn (NASA/GSFC Code 660.1) <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:tom.mcglynn@nasa.gov"
target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:tom.mcglynn@nasa.gov">tom.mcglynn@nasa.gov</a></a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Arnold,<br>
<br>
Assuming you mean for a match to occur whenever the two
circles overlap, then wouldn't<br>
<br>
xmatch(ra1,dec1,rad1, ra2,dec2,rad2)<br>
<br>
be equivalent to<br>
<br>
xmatch(ra1, dec1, ra2, dec2, rad1+rad2)<br>
<br>
if we're returning only integer values 1 and 0.<br>
<br>
I suppose one could define this differently with<br>
xmatch(ra1,dec1,rad1, ra2, dec2,rad2)<br>
returning the fraction of the circle defined by
ra1,dec1,rad1 which<br>
is enclosed in the second circle. Not sure I really see the
use case for that though.<br>
<br>
Tom<br>
<br>
<br>
Arnold Rots wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"><span
class="">
I don't think I am the only one who is not enamored of
these<br>
brute-force unnuanced cross-matches, but I wonder
whether<br>
it would be helpful to improve them by allowing two
radii.<br>
<br>
Cheers,<br>
<br>
- Arnold<br>
<br>
-------------------------------------------------------------------------------------------------------------<br>
Arnold H. Rots Chandra X-ray Science Center<br>
Smithsonian Astrophysical Observatory tel: <a
moz-do-not-send="true"
href="tel:%2B1%20617%20496%207701"
value="+16174967701" target="_blank">+1 617 496 7701</a><br>
60 Garden Street, MS 67 fax: <a
moz-do-not-send="true"
href="tel:%2B1%20617%20495%207356"
value="+16174957356" target="_blank">+1 617 495 7356</a><br>
</span>
Cambridge, MA 02138 <a moz-do-not-send="true"
href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a>
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<div class="h5"><br>
<br>
<br>
On Tue, Feb 9, 2016 at 10:21 AM, Mark Taylor <<a
moz-do-not-send="true"
href="mailto:M.B.Taylor@bristol.ac.uk"
target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:M.B.Taylor@bristol.ac.uk">M.B.Taylor@bristol.ac.uk</a></a>
<mailto:<a moz-do-not-send="true"
href="mailto:M.B.Taylor@bristol.ac.uk"
target="_blank">M.B.Taylor@bristol.ac.uk</a>>>
wrote:<br>
<br>
Hi Marco and DAL,<br>
<br>
I support the introduction of a new XMATCH
function for two main<br>
reaons:<br>
<br>
Usability:<br>
A dedicated crossmatch function should be
easier<br>
for users to use and remember, and all round
less horrible,<br>
than the current recommended(?) cross-match
idiom:<br>
1 = CONTAINS(POINT(coordsys, lon1, lat1),<br>
CIRCLE(coordsys, lon2, lat2,
radius))<br>
(where coordsys is a string that should
probably be 'ICRS', or<br>
maybe should be empty, but anyway is unlikely
to make much<br>
difference to the result).<br>
<br>
Implementability:<br>
It may make it easy/possible to provide
standard ADQL syntax for<br>
efficient sky crossmatching in database
backends that otherwise<br>
can't do it, because they have trouble
implementing the ADQL<br>
geometry functions (because they lack
pgSphere).<br>
<br>
In my opinion it should look like this:<br>
<br>
1 = XMATCH(lon1, lat1, lon2, lat2, radius)<br>
<br>
The alternative would presumably be<br>
<br>
1 = XMATCH(POINT(coordsys, lon1, lat1),
POINT(coordsys, lon2,<br>
lat2), radius)<br>
<br>
which from the point of view of usability is not
much better than<br>
the status quo. Although I believe the annoying
and disingenuous<br>
coordsys argument to the geometry functions is
scheduled for removal<br>
from ADQL, my understanding is that it's not
intended for this<br>
(minor) revision. Even without the coordsys arg,
I think the<br>
POINTless form looks less intimidating for users.<br>
<br>
I would have thought that from the point of view
of implementability<br>
as well the POINTless form presents fewer
constraints.<br>
However, I'm not a database implementation person,
so I might be<br>
wrong about that.<br>
<br>
Mark<br>
<br>
On Tue, 9 Feb 2016, Marco Molinaro wrote:<br>
<br>
> Dear DAL members and ADQL fans,<br>
> to go on with the ADQL-2.1 working draft<br>
> one issue is left, from Sydney interop,<br>
> to be discussed.<br>
><br>
> In the DAL splinter at the interop<br>
> it was agreed to add an XMATCH function<br>
> of binary type and definition<br>
><br>
> 1 = XMATCH(a,b,radius)<br>
><br>
> However no agreement was reached about<br>
> the 'a' and 'b' parameters, whether they<br>
> should be points (ADQL:POINT) or RA&Dec<br>
> couples (floating point values).<br>
><br>
> Both choices have advantages and
disadvantages.<br>
> Points are more into the logic<br>
> of a sky cross-match but require geometric<br>
> types to be directly available to the DB.<br>
> Coordinates couples are directly available<br>
> in whatever DB and would also let the XMATCH<br>
> function work for non-orthodox coordinates<br>
> matching, but of course loosing the sky
matching<br>
> logic.<br>
><br>
> As I said (also due to time constraints) no<br>
> agreement was found in Sydney.<br>
><br>
> What's your opinion on this, and why?<br>
><br>
> Cheers,<br>
> Marco<br>
><br>
<br>
--<br>
Mark Taylor Astronomical Programmer Physics,
Bristol<br>
University, UK<br>
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</blockquote>
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</blockquote>
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</blockquote>
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<br>
<pre class="moz-signature" cols="72">--
François-Xavier Pineau
CDS, Observatoire Astronomique de Strasbourg
11, rue de l'Université
F - 67000 Strasbourg</pre>
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