ggplot2 / gis在多边形区域内绘图

Question

出于复制目的，请考虑以下数据：

library(rgdal)
library(ggplot2)
library(rgeos)

download.file("http://spatialanalysis.co.uk/wp-content/uploads/2010/09/London_Sport.zip", 

destfile = "London_Sport.zip")
unzip("London_Sport.zip")

projection="+proj=merc"

london_shape = readOGR("./", layer="london_sport")

# Create random points
set.seed(1)
points = data.frame(long=rnorm(10000, mean=-0.1, sd=0.1), lat=rnorm(10000, mean=51.5, sd=0.1))
points = SpatialPoints(points, proj4string=CRS("+proj=latlon"))

# Transform data to our projection
london = spTransform(london_shape, CRS(projection))
points = spTransform(points, CRS(projection))

# Keeps only points inside London
intersection = gIntersects(points, london, byid = T)
outside = apply(intersection == FALSE, MARGIN = 2, all)
points = points[which(!outside), ]

# Blank theme
new_theme_empty <- theme_bw()
new_theme_empty$line <- element_blank()
new_theme_empty$rect <- element_blank()
new_theme_empty$strip.text <- element_blank()
new_theme_empty$axis.text <- element_blank()
new_theme_empty$plot.title <- element_blank()
new_theme_empty$axis.title <- element_blank()
new_theme_empty$plot.margin <- structure(c(0, 0, -1, -1), unit = "lines", valid.unit = 3L, class = "unit")

# Prepare data to ggplot
london = fortify(london)
points = as.data.frame(points)

我想绘制点的密度图。我可以使用stat_bin2d：

这样做

ggplot() + 
  geom_polygon(data=london, aes(x=long,y=lat,group=group), fill="black") +
  stat_bin2d(data=points, aes(x=long,y=lat), bins=40) +
  geom_path(data=london, aes(x=long,y=lat,group=id), colour='white') +
  coord_equal() +
  new_theme_empty

但是这会导致密度方块的某些部分被绘制在伦敦之外：

如何在伦敦境内绘制密度图？

Answer 1

我通过获取多边形来找到答案，该多边形是伦敦的边界框和伦敦本身（使用gDifference）之间的差异，并在所有内容上以白色绘制。我可以想到的这种方法的缺点是：1）如果一些方块仍然出现在它后面，你必须手动增加多边形的大小。 2）你不能使用复杂背景的绘图主题。如果有人有更好的答案，我会暂时搁置这个问题。

以下是代码：

library(rgdal)
library(ggplot2)
library(rgeos)

projection="+proj=merc"

#London boroughs polygons
download.file("http://spatialanalysis.co.uk/wp-content/uploads/2010/09/London_Sport.zip", destfile = "London_Sport.zip")
unzip("London_Sport.zip")
london = readOGR("./", layer="london_sport")
london = spTransform(london, CRS(projection))

# Generate random points
set.seed(1)
points = data.frame(long=rnorm(10000, mean=-0.1, sd=0.1), lat=rnorm(10000, mean=51.5, sd=0.1))
points = SpatialPoints(points, proj4string=CRS("+proj=latlon"))
points = spTransform(points, CRS(projection))

# Keep only points inside London
intersection = gIntersects(points, london, byid = TRUE)
inside = apply(intersection == TRUE, MARGIN = 2, any)
points = points[which(inside), ]

# Create a bounding box 10% bigger than the bounding box of London
x_excess = (london@bbox['x','max'] - london@bbox['x','min'])*0.1
y_excess = (london@bbox['y','max'] - london@bbox['y','min'])*0.1
x_min = london@bbox['x','min'] - x_excess
x_max = london@bbox['x','max'] + x_excess
y_min = london@bbox['y','min'] - y_excess
y_max = london@bbox['y','max'] + y_excess
bbox = matrix(c(x_min,x_max,x_max,x_min,x_min,
                y_min,y_min,y_max,y_max,y_min),
              nrow = 5, ncol =2)
bbox = Polygon(bbox, hole=FALSE)
bbox = Polygons(list(bbox), "bbox")
bbox = SpatialPolygons(Srl=list(bbox), pO=1:1, proj4string=london@proj4string)

# Get the Polygon that is the difference between the bounding box and London
outside = gDifference(bbox,london)

# Blank theme
new_theme_empty <- theme_bw()
new_theme_empty$line <- element_blank()
new_theme_empty$rect <- element_blank()
new_theme_empty$strip.text <- element_blank()
new_theme_empty$axis.text <- element_blank()
new_theme_empty$plot.title <- element_blank()
new_theme_empty$axis.title <- element_blank()
new_theme_empty$plot.margin <- structure(c(0, 0, -1, -1), unit = "lines", valid.unit = 3L, class = "unit")

# Prepare data for ggplot
london = fortify(london)
points = as.data.frame(points)
outside = fortify(outside)

# Plot!
ggplot() + 
  geom_polygon(data=london, aes(x=long,y=lat,group=group), fill="black") +
  stat_bin2d(data=points, aes(x=long,y=lat), bins=40) +
  geom_path(data=london, aes(x=long,y=lat,group=id), colour='white') +
  geom_polygon(data=outside, aes(x=long,y=lat), fill='white') +
  coord_equal() +
  new_theme_empty