After a colleague of mine requested an illustration of the aggregation technique that was part of an introduction to spatial data in R, I decided to revisit aggregation. The first article I posted here on the subject used data from London; this one is slightly different, using Twitter data aggregated to the state level in the USA. All example code and data can be downloaded from here.

In short, this vignette demonstrates the geographical aggregation of point data to create choropleth maps made using ggplot2.

Load the state data

library(rgdal)

## Loading required package: sp
## rgdal: version: 0.8-10, (SVN revision 478)
## Geospatial Data Abstraction Library extensions to R successfully loaded
## Loaded GDAL runtime: GDAL 1.10.0, released 2013/04/24
## Path to GDAL shared files: /usr/share/gdal/1.10
## Loaded PROJ.4 runtime: Rel. 4.8.0, 6 March 2012, [PJ_VERSION: 480]
## Path to PROJ.4 shared files: (autodetected)

states <- readOGR(".", "states")

## OGR data source with driver: ESRI Shapefile 
## Source: ".", layer: "states"
## with 51 features and 5 fields
## Feature type: wkbPolygon with 2 dimensions

summary(states)

## Object of class SpatialPolygonsDataFrame
## Coordinates:
##       min    max
## x -178.22 -66.97
## y   18.92  71.41
## Is projected: FALSE 
## proj4string :
## [+proj=longlat +datum=NAD83 +no_defs +ellps=GRS80 +towgs84=0,0,0]
## Data attributes:
##       STATE_NAME    DRAWSEQ       STATE_FIPS              SUB_REGION
##  Alabama   : 1   Min.   : 1.0   01     : 1   South Atlantic    : 9  
##  Alaska    : 1   1st Qu.:13.5   02     : 1   Mountain          : 8  
##  Arizona   : 1   Median :26.0   04     : 1   West North Central: 7  
##  Arkansas  : 1   Mean   :26.0   05     : 1   New England       : 6  
##  California: 1   3rd Qu.:38.5   06     : 1   East North Central: 5  
##  Colorado  : 1   Max.   :51.0   08     : 1   Pacific           : 5  
##  (Other)   :45                  (Other):45   (Other)           :11  
##    STATE_ABBR
##  AK     : 1  
##  AL     : 1  
##  AR     : 1  
##  AZ     : 1  
##  CA     : 1  
##  CO     : 1  
##  (Other):45

states <- spTransform(states, CRS("+init=epsg:4326"))
states <- states[-which(grepl("Alask|Haw", as.character(states$STATE_NAME))), 
    ]
tweets <- read.csv("1pSample.txt")
plot(states)
points(tweets$lon, tweets$lat, col = "blue")

Convert the tweets into a spatial (S4) class

tweets <- SpatialPointsDataFrame(coords = matrix(c(tweets$lon, tweets$lat), 
    ncol = 2), data = tweets, proj4string = CRS("+init=epsg:4326"))

Now lets aggregate by number of tweets.

statesAg1 <- aggregate(tweets["X"], states, length)

Aggregating by average n. friends in territory:

statesAg2 <- aggregate(tweets["actor.friendsCount"], by = states, mean)
statesAg1$friends <- statesAg2$actor.friendsCount
statesAg1$id <- as.character(states$STATE_NAME)

Visualisation

Preparing the data to plot with ggplot2.

library(ggplot2)
library(dplyr)

## 
## Attaching package: 'dplyr'
## 
## The following objects are masked from 'package:stats':
## 
##     filter, lag
## 
## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union

sf <- fortify(statesAg1, region = "id")

## Loading required package: rgeos
## rgeos version: 0.2-19, (SVN revision 394)
##  GEOS runtime version: 3.3.8-CAPI-1.7.8 
##  Polygon checking: TRUE

head(sf)

##     long   lat order  hole piece     group      id
## 1 -85.07 31.98     1 FALSE     1 Alabama.1 Alabama
## 2 -85.12 31.91     2 FALSE     1 Alabama.1 Alabama
## 3 -85.14 31.85     3 FALSE     1 Alabama.1 Alabama
## 4 -85.13 31.78     4 FALSE     1 Alabama.1 Alabama
## 5 -85.13 31.78     5 FALSE     1 Alabama.1 Alabama
## 6 -85.12 31.73     6 FALSE     1 Alabama.1 Alabama

head(statesAg1@data)

##    X friends           id
## 1  5   573.6   Washington
## 2  1     0.0      Montana
## 3  3   250.0        Maine
## 4 NA      NA North Dakota
## 5  2  1194.5 South Dakota
## 6 NA      NA      Wyoming

sf <- inner_join(sf, statesAg1@data, by = "id")
head(sf)

##     long   lat order  hole piece        group         id X friends
## 1 -122.4 48.23 10493 FALSE     1 Washington.1 Washington 5   573.6
## 2 -122.5 48.23 10494 FALSE     1 Washington.1 Washington 5   573.6
## 3 -122.5 48.13 10495 FALSE     1 Washington.1 Washington 5   573.6
## 4 -122.4 48.06 10496 FALSE     1 Washington.1 Washington 5   573.6
## 5 -122.5 48.13 10497 FALSE     1 Washington.1 Washington 5   573.6
## 6 -122.5 48.21 10498 FALSE     1 Washington.1 Washington 5   573.6

Plot.

ggplot(sf, aes(long, lat, fill = X, group = group)) + geom_polygon() + scale_fill_gradient(low = "green", 
    high = "red") + coord_map()

ggplot(sf, aes(long, lat, fill = friends, group = group)) + geom_polygon() + 
    scale_fill_gradient(low = "blue", high = "orange") + coord_map()