Bill Binko kirjoitti:
Brent,
Thanks for the info. I will certainly look at the R tools: I was
unaware of an S replacement (it's been years since I've seen S).
It does seem (from what little research I've done) that these R tools
will generate images, and not shapefiles.
I am primarily like to take Point datasets, and generate polygon
datasets. I suppose I could take R, generate rasters, and then use
gdal_contour to generate the vectors?
However, I am willing (and probably capable) of writing a tool that
can process the points, run the kernel function and generate the
resulting contours directly. Does anyone know where I can find a good
description of the algorithm and/or some sample code for it? Perhaps
one of the professors out there?
I have no experience with this (despite my status :-) but I believe the
idea at its simplest is that the effect of the points (e.g., location of
an incident, which has no area) are "spread out" using the kernel
function (which has a value over some area), and thus you get a
continuous function of incidents/unit area. The parameters of the kernel
function are of course subjective, so visualization is used to select
"the best" values. Google gave me this URL on doing spatial kernel
density estimation with R:
http://www.math.uni-klu.ac.at/~agebhard/ERSA-98-D8-427/node6.html
but I'd go to some handbooks to learn more. I think the use of rasters
for computing the density map on is a practical thing. It is easy to
compute the density at any given point but it is much more difficult to
find the points where the density gets a specific value (which you would
need to do if you want to compute the contours directly).
Ari
Of course, if I did so, I'd be happy to push it back to the community.
Bill
Brent Wood wrote:
--- Bill Binko <[EMAIL PROTECTED]> wrote:
Hi everyone,
[I've been sidelined recently battling some painful bugs in GDAL,
Mapserver, and several raster libs, so I've been putting off some
more general questions I'd love to pose to the list. This is my
first, but I'll probably send several tonight (sorry to flood).]
I'm interested in creating "kernel method" density maps using vector
point data.
Look at R for this sort of thing, I know several fisheries modellers
using R
for kernel density analyses. R/PostGIS/GRASS/QGIS/mapserver (with
Proj.4, GDAL,
GEOS, etc...) all work pretty well together to support such work as
an Open
Source suite.
See:
http://www.stat.math.ethz.ch/R-manual/R-devel/library/stats/html/density.html
For a more general list of R functions,
http://www.oulu.fi/atkk/tkpalv/unix/R/library/stats/html/00Index.html
and for general R spatial packages/info,
http://r-spatial.sourceforge.net/
Hope this helps....
Brent Wood
--
Prof. Ari Jolma
Kartografia ja Geoinformatiikka / Cartography and Geoinformatics
Teknillinen Korkeakoulu / Helsinki University of Technology
POBox 1200, 02015 TKK, Finland
Email: ari.jolma at tkk.fi URL: http://www.tkk.fi/~jolma
