How can I overlay two spatialpolygons and plot them? The following  
produces two maps since a new plot is called. I understand that if it  
is any other type, we can simply call lines or points and overlay them  
on polygons.

e.g.
library(sp)

## plot of SpatialPolygonsDataFrame, using grey shades
library(maptools)
nc1 <- readShapePoly(system.file("shapes/sids.shp", package="maptools") 
[1], proj4string=CRS("+proj=longlat +datum=NAD27"))
names(nc1)
rrt <- nc1$SID74/nc1$BIR74
brks <- quantile(rrt, seq(0,1,1/7))
cols <- grey((length(brks):2)/length(brks))
dens <- (2:length(brks))*3
plot(nc1, col=cols[findInterval(rrt, brks, all.inside=TRUE)])

library(sp)

## plot of SpatialPolygonsDataFrame, using line densities
library(maptools)
nc <- readShapePoly(system.file("shapes/sids.shp", package="maptools") 
[1], proj4string=CRS("+proj=longlat +datum=NAD27"))
names(nc)
rrt <- nc$SID74/nc$BIR74
brks <- quantile(rrt, seq(0,1,1/7))
cols <- grey((length(brks):2)/length(brks))
dens <- (2:length(brks))*3
plot(nc, density=dens[findInterval(rrt, brks, all.inside=TRUE)])

Nikhil Kaza
Asst. Professor,
City and Regional Planning
University of North Carolina

nikhil.list@gmail.com


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