John Snow, and Google Maps

February 27, 2015
By

(This article was first published on Freakonometrics » R-english, and kindly contributed to R-bloggers)

In my previous post, I discussed how to use OpenStreetMaps (and standard plotting functions of R) to visualize John Snow’s dataset. But it is also possible to use Google Maps (and ggplot2 types of graphs).

library(ggmap)
get_london <- get_map(c(-.137,51.513), zoom=17)
london <- ggmap(get_london)

Again, the tricky part comes from the fact that the coordinate representation system, here, is not the same as the one used on Robin Wilson’s blog.

library(foreign)
deaths=read.dbf(".../Cholera_Deaths.dbf")

So we have to change it

df_deaths=data.frame(deaths@coords)
library(sp)
library(rgdal)
coordinates(df_deaths)=~coords.x1+coords.x2
proj4string(df_deaths)=CRS("+init=epsg:27700") 
df_deaths = spTransform(df_deaths,CRS("+proj=longlat +datum=WGS84"))

Here, we have the same coordinate system as the one used in Google Maps. Now, we can add a layer, with the points,

london + geom_point(aes(x=coords.x1, y=coords.x2),data=data.frame([email protected]),col="red")

Again, it is possible to add the density, as an additional layer,

london + geom_point(aes(x=coords.x1, y=coords.x2), 
data=data.frame([email protected]),col="red")+
geom_density2d(data = data.frame([email protected]), 
aes(x = coords.x1, y=coords.x2), size = 0.3) + 
stat_density2d(data = data.frame([email protected]), 
aes(x = coords.x1, y=coords.x2,fill = ..level.., alpha = ..level..),size = 0.01, bins = 16, geom = "polygon") + scale_fill_gradient(low = "green", high = "red",guide = FALSE) + 
scale_alpha(range = c(0, 0.3), guide = FALSE)

 

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