[R] Help with map

[Roger Bivand]

On Fri, 4 May 2007, Alberto Monteiro wrote:

> I have just learned how to play with map, but something weird
> (or not) is happening.
> 
> Suppose I want to draw a map of two countries (that have disconnected
> components), like Argentina and Brazil.
> 
> If I command:
> 
> library(maps)
> library(mapdata)
> map("worldHires", c("Argentina", "Brazil"))
> 
> It works fine. However, if I want to _colour_ the interior:
> 
> map("worldHires", c("Argentina", "Brazil"), c("cyan", "green"), fill=T)
> 
> Then the colors will be assigned to the islands (Marajo in Brazil's
> North and Tierra del Fuego in Argentina's South) and there will be
> a recycling.
> 
> 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

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,

Roger

> 
> Alberto Monteiro
> 
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> 

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no