On Mon, 13 Dec 2004, Dylan Beaudette wrote:
Hi!
I was wondering if anyone has tried to re-classify the values of a continuous
raster image with a method known as "natural breaks' in the ESRI world, or
any other statistical technique for that matter. I was hoping to try some of
the methods talk about on this website;
http://www.geovista.psu.edu/grants/dg-qg/classing_epi/summary.html
Would it be possible to computer the class intervals in GRASS, or would this
have to be done in a stats package like R?
I've some results on this in a paper written in March using R:
http://spatial.nhh.no/papers/aag04.pdf
where the interesting bits are using stats in R to try out natural breaks
- references in the paper. The best solution to date is the R bclust()
function for bagged clustering in package e1071, because it combines the
strength of kmeans() and hiearchical clustering, allowing a post-fitting
choice of number of classes. The same method is used in the course notes
on this page: http://spatial.nhh.no/geo304/modules-h04.html - look under
week 39. The interesting geovista reference is: Armstrong, M.~P., Xiao,
N., Bennett, D.~A., 2003. {Using genetic algorithms to create
multicriteria class intervals for choropleth maps}. {Annals, Association
of American Geographers}, 93 (3), {595--623}.
Straight-in coded natural breaks run into the combinatorical problem that
there are millions of possible maps for each number of classes (above
trivially small numbers like 1 or 2).
I've found bclust() very good - if there is too little data, the trick in
the lecture notes of including a jittered replicate of the original data
usually helps. I also like displaying an empirical cumulative distribution
function to "see" the class intervals. R startup is getting better too.
With a little pushing, making a tcltk display in R run from GRASS is
doable to help choose class intervals.
A discussion on some of these cognitive issues woulkd be valuable,
Roger
Many thanks,
--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
e-mail: Roger.Bivand@nhh.no