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Re: match_2d [message #66227 is a reply to message #66220]

Tue, 28 April 2009 19:44

Jeremy Bailin
Messages: 618
Registered: April 2008

Senior Member

On Apr 27, 3:06 pm, JDS <jdtsmith.nos...@yahoo.com> wrote:
>> I'm pretty sure there's a HIST_ND-based algorithm of doing this
>> similar toMATCH_2Dbut taking spherical trig into account, but I
>> don't have the patience to figure it out.
>
> That would be challenging for the whole sphere, since histogram can
> only evaluate monotonic coordinate fields. You can always first remap
> your coordinates using some projection which puts the ill-behaved
> parts (nominally, the poles) far away, and preserves distance
> locally. For example, if you have a small field (a degree or so) near
> the pole, this would be a nice way of solving the converging longitude
> lines issues. But generally? Sounds tough.
>
> JD

How about if it was done in 3D? Instead of 2D angular coordinates, use
the 3D coordinates of the relevant points on the surface of a unit
sphere, and then use HIST_ND to determine which 3D bin the points are
in and build the algorithm analogously to MATCH_2D?

The main problem I see is that, for small bin sizes (ie. small desired
angular separations), there's a lot of wasted memory storing the
histogram in locations that don't lie on the surface of the sphere and
therefore are necessarily zero. But maybe there's a way of enumerating
the bins that do contain part of the surface - if so, then you could
use that enumeration to map the 3D positions into a simple number that
you can run HISTOGRAM on.

-Jeremy.

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[Message index]

		match_2d By: vino on Mon, 20 April 2009 06:40
		Re: match_2d By: Jeremy Bailin on Tue, 28 April 2009 19:44
		Re: match_2d By: JDS on Wed, 29 April 2009 13:29
		Re: match_2d By: JDS on Mon, 27 April 2009 12:06
		Re: match_2d By: vino on Mon, 27 April 2009 03:53
		Re: match_2d By: Jeremy Bailin on Wed, 29 April 2009 17:00
		Re: match_2d By: Jeremy Bailin on Fri, 08 May 2009 22:02
		Re: match_2d By: Jeremy Bailin on Wed, 29 April 2009 16:30
		Re: match_2d By: wlandsman on Thu, 06 May 2010 11:11
		Re: match_2d By: d.poreh on Fri, 07 May 2010 05:06
		Re: match_2d By: d.poreh on Thu, 06 May 2010 05:25
		Re: match_2d By: vino on Thu, 06 May 2010 04:27
		Re: match_2d By: d.poreh on Fri, 07 May 2010 05:55
		Re: match_2d By: Jeremy Bailin on Fri, 07 May 2010 05:30

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