[R] Help with map

Alberto Vieira Ferreira Monteiro albmont at centroin.com.br
Sat May 5 03:00:16 CEST 2007

[for those that worry about these things, this _is_ a homework
assignment. However, it's not an R homework, it's a Geography
and History homework... and I want to use R to create a pretty

Roger Bivand wrote:
>> Is there any way to associate one color to each country?
> Try:
> map_poly_obj <- map("worldHires", c("Argentina", "Brazil"), plot=FALSE,
>   fill=TRUE)
> str(map_poly_obj)
> and you'll see that the component of interest is the named polygons, of
> which there are 28, namely
Ok, I guess I can see what you mean. It worked, but I don't think
this is a practical way to draw things.

For example, suppose [this would help homework mentioned above] I
want to draw a series of maps showing the evolution of Communism
in the XX century. I would like to start with a 1917 map, showing most
countries as in...


... but with the Soviet Union in red. I don't see how I could mix the two 
maps (BTW, there's no Russia in worldHires, but there is a USSR...)

map("worldHires"); map("worldHires", "USSR", col="red", fill=T)

> map_poly_obj$names
> So you can build a matching colours vector, or:
> library(sp)
> library(maptools)
> IDs <- sapply(strsplit(map_poly_obj$names, ":"), function(x) x[1])
> SP_AB <- map2SpatialPolygons(map_poly_obj, IDs=IDs,
>   proj4string=CRS("+proj=longlat +datum=wgs84"))
> but
> plot(SP_AB, col=c("cyan", "green"))
> still misses, because some polygons have their rings of coordinates in
> counter-clockwise order, so:
> pl_new <- lapply(slot(SP_AB, "polygons"), checkPolygonsHoles)
> slot(SP_AB, "polygons") <- pl_new
> # please forget the assignment to the slot and do not do it unless you can
> # replace what was there before
> plot(SP_AB, col=c("cyan", "green"), axes=TRUE)
> now works. Moreover, SP_AB is a SpatialPolygons object, which can be
> promoted to a SpatialPolygonsDataFrame object, for a data slot holding a
> data.frame with row names matching the Polygons ID values:
> sapply(slot(SP_AB, "polygons"), function(x) slot(x, "ID"))
> So adding a suitable data frame gets you to the lattice graphics method
> spplot(SP_AB, "my_var")
> Hope this helps,
So, in the above mentioned case, I could do something like:

commies <- c("USSR", "Mongolia") 
# Mongolia was the 2nd communist country, in 1925
map_poly_obj <- map("worldHires", plot=FALSE)
map_poly_commies <- map("worldHires", commies,
  plot=FALSE, fill=TRUE)
plot(map_poly_obj, type="l")
polygon(map_poly_commies, col="red", border="black")

I guess I can keep going, unless there is a simpler solution.

Alberto Monteiro

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