Help with polygons

[Joe]

Hi All,

My goal is to create a Voronoi Tessellation which is bounded by the
borders of the United States and Canada, then export the polygons to a
dataframe for plotting in ggplot2. After much futzing around with
spatstat and sp, I've given up on figuring out how to do this the
right way. I got this 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)
us.d <- dirichlet(us.ppp)

Extracting polygons from the output of dirichlet() seems hopeless.

So, I'm wondering if I can fake it. I can create a voronoi
tessellation from my data with deldir(), and get that into the right
shape for plotting. I'm wondering if there is some way to plot the
inverse of the us as a polygon, plotted over the voronoi tessellation,
making it appear to be bounded by the borders of the us.

us$ID <- 1
ggplot(us, aes(x,y)) + geom_polygon(aes(group = ID))

-Joe

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[Andrie de Vries]

Joe

Is the function unionSpatialPolygons() in the maptools package of any
use?

http://bm2.genes.nig.ac.jp/RGM2/R_current/library/maptools/man/unionSpatialPolygons.html

There is a worked example of using this function here:

http://www.nceas.ucsb.edu/files/scicomp/GISSeminar/UseCases/PolygonDissolve/PolygonDissolve.html

Andrie

[Joe]

Thanks for those links, but I ended up going in a different direction
using the gpclib library.

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(deldir)
library(gpclib)

us <- data.frame(map("usa", regions = "main",plot = F)[c("x","y")])
manhattan <- data.frame(map("usa", regions = "manhattan",plot = F)
[c("x","y")])

longisland <- data.frame(map("usa", regions = "long island",plot = F)

[c("x","y")])

cities.tess <- deldir(cities$Longitude, cities$Latitude)

clip.border <- function(deldir.obj, border){
require(gpclib)
bord <- as(matrix(ncol = 2), "gpc.poly")
for(i in seq(along = border)){
b <- as(border[[i]], "gpc.poly")
bord <- union(b, bord)
}

tiles <- tile.list(deldir.obj)
out <- {}
group <- 1
for(i in seq(along = tiles)){
poly <- cbind(tiles[[i]]$x, tiles[[i]]$y)
poly <- as(poly, "gpc.poly")

poly <- intersect(poly, bord)
if(length(poly@pts)>0){
for(j in seq(along=poly@pts)){
poly.df <- data.frame(x = poly@pts[[j]]$x, y =poly@pts[[j]]$y)
poly.df$ID <- i
poly.df$group <- group
group <- group + 1
out <- rbind(out, poly.df)
}
}
}
return(out)
}

tess.df <- clip.border(cities.tess, list(us, manhattan, longisland))
tess.df$CitState <- cities[tess.df$ID,]$CitState

ggplot(tess.df, aes(x,y)) + geom_polygon(aes(group = group,fill =
CitState))+coord_map()

On May 14, 6:03 pm, Andrie de Vries <apdevr...@gmail.com> wrote:
> Joe
>
> Is the function unionSpatialPolygons() in the maptools package of any
> use?
>

> http://bm2.genes.nig.ac.jp/RGM2/R_current/library/maptools/man/unionS...

>
> There is a worked example of using this function here:
>

> http://www.nceas.ucsb.edu/files/scicomp/GISSeminar/UseCases/PolygonDi...