Remove points outside a map with ggplot2

Question

My dataset consist in a converted raster dataframe, with for each point a long/lat, a categorical value and a numerical value are associated with. It can be downloaded here.

The code hereafter allows me to generate this map.

My issue is that I can't find a way to get rid of all the points that are mapped outside the polygons boundaries. I tried to convert the df as a raster and use the raster::mask function, but when doing this I loose information. I also tried the the rgeos::gDifference, but same problem, can't make the job. Any hint on how I can achieve my goal?

library(rgdal) 
library(raster)
library(sp)
library(maptools)
library(ggplot2)
library(ggmap)
# load spatial data
fr <- getData('GADM', country = 'France', level = 0)
fr <- spTransform(fr, CRS("+proj=longlat +datum=WGS84")) #provide a 
coordinate system
#open df to plot
df = read.table("mapdata.txt", header=T, sep="\t", quote="", dec=".")
ggplot() +
 geom_tile(data=df, aes(x=lon, y=lat, fill=item, alpha=prob)) +
 geom_polygon(data = fr, aes(x=long, y = lat, group = group), 
 fill="transparent", colour = "white", size=1) +
 theme_nothing(legend = FALSE) 

• Spacedman: What do you mean you lose information when using raster::mask?
• Spacedman: My R wasn't too happy with your data file. Linux thinks its ISO-8859 encoded and it would be much happier if it was UTF-8.

Answer 1

Personally I'd ditch ggplot and use raster as much as possible.

But here's a solution.

Work out which points are in and which are out of France:

inout = over(
 SpatialPoints(df[,c("lon","lat")],proj4string=CRS(projection(fr))),
 as(fr,"SpatialPolygons")
)

Then ggplot the subset that isn't NA:

> ggplot() +
 geom_tile(data=df[!is.na(inout),],aes(x=lon, y=lat, fill=item, alpha=prob)) + 
 geom_polygon(data = fr, aes(x=long, y = lat, group = group), 
 fill="transparent", colour = "white", size=1) +
 theme_nothing(legend = FALSE) 

enter image description here