Creating a Cartogram in R

Question

I am trying to make a cartogram in R to show the number of occurrences in each area of the UK.

My data currently looks like this:

Area                           Occurences        lon      lat
1 Greater London East North UK        200 -0.0936496 51.43092
2     Lambeth and Southwark UK        16 -0.1178424 51.49351
3             Black Country UK        58 -2.0752861 52.52005
4                   Glasgow UK        45 -4.2518060 55.86424
5                     Leeds UK        331 -1.5490774 53.80076
6     Sth Herts or  Watford UK        210 -0.3903200 51.65649

I have the longitude and latitude for all of the 120 observations. So far I have used the following code in an attempt to produce a cartogram:

library(rgdal)
library(cartogram)
library(tmap)
library(maptools)

ukgrid = "+init=epsg:27700"

data(wrld_simpl)

afr <- wrld_simpl[wrld_simpl$NAME == "United Kingdom",]
afr <- spTransform(afr, CRS(ukgrid))

# construct cartogram
afrc <- cartogram(afr, "POP2005", itermax=5)

# plot it
tm_shape(afrc) + tm_fill("POP2005", style="jenks") + 
tm_borders() + tm_layout(frame=F)

This produces the UK map but I am unsure how to use my own data for the cartogram as opposed to the population data in the 'wrld_simpl' data that the map is based on.

Does anyone have any experience of doing this or know another method to achieve the desired result? Thanks!

Answer 1

I assume your data is a SpatialPointsDataFrame? You'll need to find a proper shapefile (SpatialPolygonsDataFrame) where each UK area corresponds to a polygon. A good source for shapes is http://www.naturalearthdata.com/

This is an example that should be similar to your case:

library(rgeos)
library(sp)
library(maptools)
library(tmap)
library(tmaptools)
library(cartogram)

data(wrld_simpl)
data(metro)

## count occurences per polygon: in this case, the number of cities per country
x <- over(metro, wrld_simpl)
res <- table(x$ISO3)
dat <- data.frame(iso_a3=names(res), count=as.vector(res))

## add counts to polygon shape
wrld_simpl <- append_data(wrld_simpl, dat, key.shp = "ISO3", key.data = "iso_a3", ignore.na = TRUE)

## remove 0 counts
wrld_simpl_sel <- wrld_simpl[which(wrld_simpl$count>0), ]

## apply cartogram (doesn't result in a nice cartogram because the shape is too detailed and the counts are too few)
wrld_simpl_carto <- cartogram(wrld_simpl_sel, weight = "count", itermax = 1)

## plot it
qtm(wrld_simpl_carto)