Scale and north arrow for maps in R

1 minute read

A few months ago I produced some thematic maps of Bosnia (paper) using maptools and other packages in R, but I didn’t include scales or a north arrow. It sounds simple and sp has functions for doing those things, but I couldn’t get it to work well with my maps. Here is a basic map of Bosnia’s pre-war municipalities:

library(maptools)

plot(bosnia)  

The function map.scale() from the maps package adds a scale. The position is in map units, latitude/longitude in this case:

library(maps)  
map.scale(x=15.5, y=42.75, ratio=FALSE, relwidth=0.2)  

And GISTools north.arrow for the north arrow. Units are also in map units. This package has a map scale function as well, which looks nicer but is a little bit more complicated to set up.

library(GISTools)  
north.arrow(xb=15.75, yb=43.25, len=0.05, lab="N")  

This will produce the following map:

Bosnia municipalities 2

Adding these to a function I wrote for producing thematic maps of Bosnia produces this pretty nice map (with scale and north arrow!) of document per capita civil war deaths:

Map of dead in Bosnia

Here is the function (which is pretty specific to the data I use):

ThematicMap <- function(vector, breaks, title, legend) {  
  require(maptools)  
  require(shape)  
  require(RColorBrewer)  
  require(GISTools)  
  require(maps)

  plotvar <- unlist(vector)  
  nclr <- 9  
  plotclr <- brewer.pal(nclr, "Reds")  
  fillRed <- colorRampPalette(plotclr)  
  plotvar[plotvar >= maxy] <- maxy -1  
  colcode <- fillRed(maxy)[round(plotvar) + 1]  
  plot(bosnia, col = colcode, lty = 0, border = "gray")  
  plot(bosnia.st, add=TRUE, lwd=1, border = "gray30")  
  plot(bosnia.front93, add = TRUE, lty="solid", lwd=1.5, col="darkblue")  
  map.scale(x=15.5, y=42.75, relwidth=0.2, ratio=FALSE)  
  north.arrow(xb=15.75, yb=43.25, len=0.05, lab="N")  
  title(main = title)  
  colorlegend(posy = c(0.05,0.9), posx = c(0.9,0.92),  
  col = fillRed(maxy),  
  zlim=c(0, maxy), zval = breaks,  
  main = legend,  
  main.cex = 0.9)  
  par(bg='white')  
}

killed.pk <- (killed/pop91*1000)  
maxy <- 50  
breaks <- c(0, 10, 20, 30, 40, Inf)  
png("images/map_dead.png")  
print(ThematicMap(killed.pk,breaks,"Documented killings during the Bosnian War","per 1,000"))  
dev.off()  

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