[AstroPy] Re : Cross matching two source lists

Jerome Caron jerome_caron_astro at ymail.com
Fri May 11 06:32:01 EDT 2012

One option could be to transform the (alpha,delta) with the projection law described here, pp5-6

PS: my apologies Thomas, I sent the email too fast to you but not to the list.

De : Thomas Robitaille <thomas.robitaille at gmail.com>
À : Sergio Pascual <sergiopr at fis.ucm.es> 
Cc : astropy mailing list <astropy at scipy.org> 
Envoyé le : Vendredi 11 mai 2012 12h04
Objet : Re: [AstroPy] Cross matching two source lists

Hi Sergio,

Thanks for your reply! So it looks like your algorithm only works for
e.g pixel or arcsec coordinates, not e.g. longitude/latitude or
ra/dec, correct? (since for spherical coordinates, even for a small
field of view, the spherical angular distance is not the same as the
euclidean distance, i.e. 1 second of RA is not 15 arcseconds except at
the equator).

For a small field of view, one could do a KDTree using (RA * cos(Dec),
Dec), but I'm interested in finding an algorithm that works for large
areas of sky.


On 11 May 2012 11:10, Sergio Pascual <sergiopr at fis.ucm.es> wrote:
> Well, I don't know if it's exactly what you are looking for, but,
> after some discussion in this list I created a package that
> implementsthe Groth algorithm for catalogue matching:
> A Pattern-Matching Algorithm for Two-Dimensional Coordinate Lists
> Eduard J. Groth 1986 AJ 19, 5
> http://adsabs.harvard.edu/abs/1986AJ.....91.1244G
> The code is here: https://bitbucket.org/sergiopr/gmatch
> Regards
> 2012/5/11 Thomas Robitaille <thomas.robitaille at gmail.com>:
>> Hi everyone,
>> I am looking for a Python package that can do arbitrary and efficient
>> cross-matching of source lists, i.e. some kind of kd-tree that uses
>> spherical rather than euclidean distances. Is anyone aware of such a
>> package?
>> Thanks,
>> Tom
>> _______________________________________________
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> --
> Sergio Pascual     http://guaix.fis.ucm.es/~spr    +34 91 394 5018
> gpg fingerprint: 5203 B42D 86A0 5649 410A F4AC A35F D465 F263 BCCC
> Departamento de Astrofísica -- Universidad Complutense de Madrid (Spain)
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It is shape-preserving, and I understand size distorsions are small even for large fields.
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