[postgis-users] quantiles, quartiles, or jenks natural

[David Bitner]

I ended up jumping into the PL/R world and just created an aggregate
wrapper around kmeans to get my class values. They ended up being
very, very close (identical in some cases) to classifications that had
been done with Jenks Natural Breaks.  If you want the same results
every time you run a classification on the same data, you need to set
the same seed value for the random number generator before each run.

It's pretty basic and my code is ugly due to some R parser errors that
I could only get passed by throwing all the code on one line with no
spaces (hey it worked and I didn't have time to look into the parser
error), but I can throw the code up if anyone would like.

On 3/2/06, Robert Burgholzer <rburghol at chesapeakebay.net> wrote:
> OK,
> I'm coming into this late, but I am a user of PL/R and PostGIS, and
> would appreciate any progress on developing some classification routines
> to be posted to this lists, or I would be interested in being notified
> offline.
>
> Thanks!
>
> r.b.
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Amit
> Kulkarni
> Sent: Wednesday, March 01, 2006 1:20 PM
> To: postgis-users at postgis.refractions.net
> Subject: Re: [postgis-users] quantiles, quartiles, or jenks natural
>
> Sorry, I have been catching up on the past few months emails. I just
> want to add that I read that quantiles and minimum boundary error are
> better than jenks. Also minimum boundary error takes into account the
> underlying topology.
>
> The two being better are mentioned in
>
> Brewer, Cynthia A. & Pickle, Linda (2002) Evaluation of Methods for
> Classifying Epidemiological Data on Choropleth Maps in Series.
> Annals of the Association of American Geographers 92 (4), 662-681
>
> And the minimum boundary algorithm is supposedly mentioned in
>
> Cromley, E. K. , and R. G. Cromley. 1996. An analysis of alternative
> classification  schemes  for  medical  atlas mapping. European Journal
> of Cancer 32A (9): 1551 -- 59.
>
> Cromley, R. G. , and R. D. Mrozinski. 1999. The classification of
> ordinal data for choropleth mapping. The Cartographic Journal 36 (2):
> 101 -- 9.
>
> HTH,
> amit
>
>
> Date: Tue, 14 Feb 2006 12:38:39 -0800
> From: Paul Ramsey <pramsey at refractions.net>
>
> I did some in PHP, but the algorithms are relatively braindead, the
> quantile stuff in particular.  Jenks I did some research on but never
> really found a definitive description of the process.  Some of the
> descriptions ended up sounding like a k-means clustering idea, which
> is not cheap!
>
> P.
>
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