[R-sig-Geo] Help with Voronoi Tesselation

[Carson Farmer]

Does this help at all?
http://www.carsonfarmer.com/?p=455

Carson

On Fri, May 14, 2010 at 1:39 AM, Josef Fruehwald <jofrhwld at gmail.com> wrote:
> Hi All,
>
> I'm somewhat of a novice at doing spatial analysis in R, so forgive me if my
> question is obvious, or misposted.
>
> I have a number of points located within the United States. My goal is to
> compute the voronoi tessellation, bounded by the borders of the US. I would
> like to eventually format this as a dataframe of coordinates with a grouping
> ID column so that I can plot the polygons in ggplot2.
>
> Here's what I've got so far.
>
> cities <- structure(list(CitState = c("SiouxFalls SD", "Rockford IL",
> "Kenosha WI",
> "Duluth MN", "SiouxCity IA", "CedarRapids IA", "NewYork NY"),
>    Latitude = c(43.54599, 42.265973, 42.577791, 46.779135, 42.489678,
>    41.976662, 40.756054), Longitude = c(-96.731291, -89.086667,
>    -87.822644, -92.108243, -96.404948, -91.673155, -73.986951
>    )), .Names = c("CitState", "Latitude", "Longitude"), row.names = c(1L,
> 2L, 3L, 4L, 5L, 6L, 267L), class = "data.frame")
>
> library(maps)
> library(spatstat)
> us <- data.frame(map("usa", regions = "main",plot = F)[c("x","y")])
> manhattan <- data.frame(map("usa", regions = "manhattan",plot =
> F)[c("x","y")])
> spatstat.options(checkpolygons = FALSE)
> us.win <- owin(poly = list(list(x = us$x, y = us$y),list(x = manhattan$x,y =
> manhattan$y)))
> us.ppp <- ppp(cities$Longitude, cities$Latitude, window = us.win)
>
> But, if I use dirichlet() on us.ppp, it gives me back a pixel array. If I
> use deldir() on us.ppp, the boundaries of the tesselations don't seem to be
> constrained by the US borders, and I can't figure out how to extract the
> polygons from the object.
>
> plot(dirichlet(us.ppp))
> ##
> map("usa")
> plot(deldir(us.ppp), wlines = "tess",add = T)
>
> If I could extract from some object, a list of polygons, I could probably
> get the rest done myself.
>
> Thanks in advance!
>
> -Joe
>
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>
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-- 
Carson J. Q. Farmer
ISSP Doctoral Fellow
National Centre for Geocomputation
National University of Ireland, Maynooth,
http://www.carsonfarmer.com/