[postgis-users] Voronoi / Dalaunay function with R

[Mike Leahy]

Jan,

Thanks - that works pretty well.  It's certainly way faster than my
script.  However, the polygons aren't quite right around the outlying
area when I tried it with one of my datasets.  In the image attached,
you can see an example of the voronoi polygons from my script (the
bigger darker polygons), and from the R-generated polygons using your
script below (the smaller lighter polygons).  You'll notice that in one
case, there's a point that isn't even inside its own voronoi polygon
because the output from R got clipped off at too short of a distance.
I'm pretty new to R (i.e., this is the first time I've ever done
anything besides install it), so I'm not sure how this could be
resolved.  Do you have any suggestions?  I'm thinking maybe I could add
four outlying corner-points located well-beyond the extent of the
dataset to force the R calculations to fill-in the entire dataset, then
clip the result to a given buffer distance.

Also, there is a slight variation in the accuracy of the vertices - from
 what I can tell, R is rounding off the coordinates of the vertices to
one decimal point, whereas the sql in the script I posted does no
rounding/snapping at all.  Would anyone know of a way to get around
that?  I guess that's really a question for the R-project list.  Looks
like I have a new learning curve to face here...

Thanks again,
Mike

Jan Hartmann wrote:
> As an alternative to Mike Leahy's and Mark Fenber's Voronoi postings:
> 
> There is an easy way to generate Voronoi polygons for PostGIS using R
> (wwww.r-project.org). In its simplest form you generate a text file with
> x-y coordinates from PostGIS, read these into R, compute the Voronoi
> diagram, and write the resulting polygons out as a text file with SQL
> insert commands:
> 
> - write out x-y-values to "points.txt"
>   psql <database>
>   \o points.txt
>   \t
>   select x,y from table
>   \o
> 
> - The Voronoi package "deldir"
>   (http://cran.r-project.org/src/contrib/Descriptions/deldir.html) is
>   not part of the standard R distribution, so you have to install it
>   with:
>   "R CMD INSTALL deldir" (for Windows, install from the "packages" menu)
> 
> - R
>   library(deldir)
>   points = scan(file="points.txt",what=list(x=0.0,y=0.0),sep="|")
>   voro = deldir(points$x,points$y)   # generate voronoi edges
>   tiles = tile.list(voro)            # combine edges into polygons
>   sink("voronoi.sql")                # redirect output to file
>   for (i in 1:length(tiles)) {       # write out polygons
>     tile = tiles[[i]]
>     cat("insert into mytable (the_geom) values(geomfromtext('POLYGON((")
>     for (j in 1:length(tile$x)) {
>        cat (tile$x[[j]],' ',tile$y[[j]],",")
>     }
>     cat (tile$x[[1]],' ',tile$y[[1]]) #close polygon
>     cat ("))',4326));\n")             # add SRID and newline
>   }
>   sink()                              # output to terminal
> 
> - In psql create a table "mytable" with a geometrycolumn "the_geom" and
> insert "voronoi.sql" (\i voronoi.sql). If you want additional
> information for each polygon, add variables to "points.txt" and adapt
> the "scan" statement accordingly. An additional variable (e.g. "id") can
> be accessed in the loop as points$id[[i]]
> 
> If you have PL/R installed, you can access everything from within
> PostgreSQL. In that case there is no need for intermediate ascii-files:
>  data can be read and written directly with "pg.spi.exec(SQL-statement)".
> 
> PostgreSQL data can also be read directly into R by means of the Rdbi
> and Rdbi.PgSQL packages
> (http://www.bioconductor.org/repository/release1.5/package/html/Rdbi.html
> and
> http://www.bioconductor.org/repository/release1.5/package/html/RdbiPgSQL.html)
> 
> They are only available for Unix and can be somewhat tricky to install.
> Alternatively, you can use the RODBC package.
> 
> Whatever method you choose, text-files, Pl/R or Rdbi/RODBC, the
> combination of PostgreSQL and R will give a huge usability boost to your
> PostGIS applications.
> 
> Jan
> 
> Jan Hartmann
> Department of Geography
> University of Amsterdam
> 
> 
> 
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> 
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