Map Attribute Data in R using GGPLOT2

Click IrishRepublicCounties for PDF version of the above above map

In this post I map some attribute data using the R software package GGPLOT2. The code primarily comes from James Cheshire excellent GIS site, Spatial Analysis. This site offers many resources, including a number of step-by-step tutorials on introductory and advanced GIS mapping in R. A couple tutorials I found useful describe how to map attribute data in R. His code is concise and well written, but doesn’t explicitly describe how to map observations made on a continuous attribute variable at discrete levels. This is something standard with ArcGIS software and probably something people want to do, or, at least, are use to doing. Think of this code as a mix between Cheshire’s posts Creating a Map with R and Creating a map with R using ggplot2.

In the following code I essentially take the observed values on continuous attribute variable, break them into intervals and plot them on a map of Irish Republic counties. The continuous variable used here was broken into five categories, though the code is modifiable enough to accommodate different numbers of break points. You’ll need to use the R packages maptools, ggplot2, RColorBrewer and classInt the for this one.

# Identify and save 5 lower and 1 upper break values
# i.e. [),[),[),[),[]
brks <- classIntervals(contVariable,n=5,style="quantile")    
# Save and extract break values       												
# Round values to two decimals
brks <- round(brks$brks,digits=2) 						

# Save categorical break value for each observed values of the continuous variable
catVariable <- findInterval(contVariable,brks,all.inside=TRUE)

# Add attribute data to SpatialPolygonsDataFrame shapefile object
counties <- spCbind(counties,catVariable)

# Make SpatialPolygonsDataFrame shapefile object compatible with GGPLOT2
# The function poly_coord_function.r is available through Cheshire's "Creating a map with R using ggplot2" post
names(counties)[1] <- "ID"
counties_geom <- poly_coords(counties)

# Plot and save map
map <- qplot(PolyCoordsY,PolyCoordsX,data=counties_geom,group=Poly_Name,fill=catVarible, geom="polygon")

# Create labels from break values
intLabels <- matrix(1:(length(brks)-1))
for(i in 1:length(intLabels )){intLabels [i] <- paste(as.character(brks[i]),"-",as.character(brks[i+1]))}

# Re-Map data
# Include a categorical legend
# Add the continuous break point label to legend				
map + scale_fill_gradientn(colours=brewer.pal(5, "Set2"),guide="legend",label=intLabels,name="contVariable name",min(contVariable),max(contVariable))

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