[R-sig-Geo] overlay points and polygons and show maps

[Takatsugu Kobayashi]

Hi,

I am trying to show grid cells with colors. Colors depends on the number 
of points falling within grid cells. Here is what I did;


rm(list=ls(all=T))
library(sp)
library(maptools)
library(PBSmapping)

### Define parameters for a grid
start.point <- -5
cco <- c(start.point,start.point)
csize <- c(1,1)
n <- abs(cco[1]*2)+1
cnum <- c(n,n)
grd <- GridTopology(cco, csize, cnum)
r.grd <- as(grd, "SpatialPolygons")
r.grd.ps <- SpatialPolygons2PolySet(r.grd)

### Plot random points
x <- rnorm(10000)
y <- rnorm(10000)
events <- as.EventData(data.frame(EID = 1:10000, X=x, Y=y))

### Count # of points within cells
n_in_cells <- findPolys(events, r.grd.ps)
n_cells <- aggregate(n_in_cells, list(n_in_cells[,2]), sum)[,c(1,4)]

Then I would like to add n_cells to r.grd.ps, and show maps with colored 
cells using plotPolys(). Any suggestions?

Thank you very much

tk