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

Stephen Woodbridge woodbri at swoodbridge.com
Thu Mar 2 07:41:25 PST 2006


Please post it to the listserv, I would be interested also. I have yet 
to jump into PL/R but it is on my list to do.


David Bitner wrote:
> 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:
>>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
>>-----Original Message-----
>>From: postgis-users-bounces at postgis.refractions.net
>>[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Amit
>>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.
>>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!
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