[R] Help with map

[Roger Bivand]

On Sat, 5 May 2007, Alberto Vieira Ferreira Monteiro wrote:

> [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
> map]
> 
> 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...
> 
> map("worldHires")
> 
> ... 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)

[Please note that the worldHires database refers to a particular time
cross section, probably late 1980's. The territorial extents of the former
USSR in 1919, 1920, 1939, 1940, 1941, 1944, 1945, etc., etc. are not the
same;  the same consideration would apply to PRC's actual control over
Tibet. 

So to do this, you need a sequence of maps showing the marginal
increments, with 1917 actually only colouring Petrograd/St Petersburg and
perhaps some other cities. I'm not aware of any publically available
sequence of boundary files adequately representing the situation of say
the Baltic states or Finland for the 1917-2007 period, if anyone has a
suitable link, please say so.

Geographical data are vintaged, not just "where", but "where when". Was
for example Estonia occupied by the USSR 1940-1941, 1944-1991, or was it
part of the USSR for the purposes of this exercise? Using disputed 
boundaries implies a choice of point of view, one that may not be 
intended.]

Roger

> 
> >
> > 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:
> 
> library(mapdata)
> 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
> 

-- 
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