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<div class="moz-cite-prefix">I agree with Marco and Mark, so just a
last comment on this thread.<br>
<br>
On 02/09/2016 06:49 PM, Arnold Rots wrote:<br>
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cite="mid:CAJXToE9MqXmABOAB0R+Etz8f5Wd_mMhOEY0GyOMr0Ke_eMcRPA@mail.gmail.com"
type="cite">
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<div>That would be nice, but I suspect that people might
find it too complicated.<br>
</div>
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</blockquote>
I guess most astronomers are used to the chi-square test so<br>
it should not be that complicated to understand.<br>
We just have to show them how it can be done iteratively<br>
when cross-correlating more than 2 catalogues <br>
(weighted mean positions and associated errors is the answer).<br>
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cite="mid:CAJXToE9MqXmABOAB0R+Etz8f5Wd_mMhOEY0GyOMr0Ke_eMcRPA@mail.gmail.com"
type="cite">
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<div>
<div>Besides, the you really would want to do a proper
Bayesian cross-match<br>
</div>
that handles complete collections of sources and takes into
account the<br>
</div>
areas of coverage as well.<br>
</div>
</div>
</blockquote>
Well, although it may not be really clear in my mind,<br>
for me the cross-match is just the selection of candidates<br>
(based on a chi-square test if we account for the positional errors,
fixing a significance level).<br>
Then, computing and putting threshold on Bayesian
probabilities/posterior likelihoods,<br>
performing cuts based on color-color diagrams/... is part of the
cross-identification<br>
and can be made in a second step.<br>
<br>
fx<br>
<br>
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cite="mid:CAJXToE9MqXmABOAB0R+Etz8f5Wd_mMhOEY0GyOMr0Ke_eMcRPA@mail.gmail.com"
type="cite">
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<div><br>
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- Arnold<br>
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<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
<a moz-do-not-send="true"
href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>
USA <a
moz-do-not-send="true"
href="http://hea-www.harvard.edu/%7Earots/"
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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<div class="gmail_quote">On Tue, Feb 9, 2016 at 12:23 PM,
François-Xavier Pineau <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:francois-xavier.pineau@astro.unistra.fr"
target="_blank"><a class="moz-txt-link-abbreviated" href="mailto:francois-xavier.pineau@astro.unistra.fr">francois-xavier.pineau@astro.unistra.fr</a></a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
<div>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
<div>
<div class="h5"><br>
<br>
On 02/09/2016 06:01 PM, Arnold Rots wrote:<br>
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<div class="h5">
<blockquote type="cite">
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<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>
<div dir="ltr">-------------------------------------------------------------------------------------------------------------<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>
Cambridge, MA 02138
<a moz-do-not-send="true"
href="mailto:arots@cfa.harvard.edu"
target="_blank">arots@cfa.harvard.edu</a><br>
USA
<a moz-do-not-send="true"
href="http://hea-www.harvard.edu/%7Earots/"
target="_blank">http://hea-www.harvard.edu/~arots/</a><br>
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<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">tom.mcglynn@nasa.gov</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> 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"><a class="moz-txt-link-abbreviated" href="mailto:arots@cfa.harvard.edu">arots@cfa.harvard.edu</a></a>
<mailto:<a moz-do-not-send="true"
href="mailto:arots@cfa.harvard.edu"
target="_blank">arots@cfa.harvard.edu</a>><br>
USA <a moz-do-not-send="true"
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rel="noreferrer" target="_blank">http://hea-www.harvard.edu/~arots/</a>
<<a moz-do-not-send="true"
href="http://hea-www.harvard.edu/%7Earots/"
rel="noreferrer" target="_blank">http://hea-www.harvard.edu/%7Earots/</a>><br>
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<div><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">M.B.Taylor@bristol.ac.uk</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>
</div>
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</blockquote>
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<span class="HOEnZb"><font color="#888888">
<pre cols="72">--
François-Xavier Pineau
CDS, Observatoire Astronomique de Strasbourg
11, rue de l'Université
F - 67000 Strasbourg</pre>
</font></span></div>
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</blockquote>
<br>
<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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