使用R的邻近地图

Question

I'm looking to create some proximity maps using R, which show how far areas are from certain points. I can't find any examples in R code, but I've found an output which is the sort of thing I want:

It doesn't necessarily have to have all the labelling/internal boundaries wizardry, but I'd like it to stop at the sea border (thinking of using the rgeos function gintersection - see here ).

I've tried doing a density plot as 'heatmaps' (this would be a pretty good solution/alternative) and putting a shapefile over the top (following this suggestion , but they're not lining up and I can't do a gintersection , probably because there's not a coordinate system attached to the density plot.

Answer 1

I used your question to play a little with new libraries...

Get a UK map and define random points

library(raster)
library(sf)
library(ggplot2)
library(dplyr)
library(tidyr)
library(forcats)
library(purrr)

# Get UK map
GBR <- getData(name = "GADM", country = "GBR", level = 1)
GBR_sf <- st_as_sf(GBR)

# Define 3 points on the UK map
pts <- matrix(c(-0.4966766, -2.0772529, -3.8437793, 
                51.91829, 52.86147, 56.73899), ncol = 2)
# Project in mercator to allow buffer with distances
pts_sf <- st_sfc(st_multipoint(pts), crs = 4326) %>% 
  st_sf() %>%
  st_transform(27700)

ggplot() +
  geom_sf(data = GBR_sf) +
  geom_sf(data = pts_sf, colour = "red")

Calculate buffer areas

We create a list of multipolygons for each buffer distance. The point dataset must be in projected coordinates (here mercator) as buffer distance is in the scale of the coordinates system.

# Define distances to buffer
dists <- seq(5000, 150000, length.out = 5)
# Create buffer areas with each distances
pts_buf <- purrr::map(dists, ~st_buffer(pts_sf, .)) %>%
  do.call("rbind", .) %>% 
  st_cast() %>%
  mutate(
    distmax = dists,
    dist = glue::glue("<{dists/1000} km"))
# Plot: alpha allows to see overlapping polygons
ggplot() +
  geom_sf(data = GBR_sf) +
  geom_sf(data = pts_buf, fill = "red",
          colour = NA, alpha = 0.1)

Remove overlapping

Buffer areas are overlapping. On the figure above, the more intense red color is due to multiple overlapping layers of transparent red. Let's remove the overlapping. We need to remove from larger areas, the buffer with the lower size. I then need to add again the smallest area to the list.

# Remove part of polygons overlapping smaller buffer
pts_holes <- purrr::map2(tail(1:nrow(pts_buf),-1),
            head(1:nrow(pts_buf),-1),
            ~st_difference(pts_buf[.x,], pts_buf[.y,])) %>%
  do.call("rbind", .) %>% 
  st_cast() %>%
  select(-distmax.1, -dist.1)
# Add smallest polygon
pts_holes_tot <- pts_holes %>% 
  rbind(filter(pts_buf, distmax == min(dists))) %>%
  arrange(distmax) %>%
  mutate(dist = forcats::fct_reorder(dist, distmax))
# Plot and define color according to dist
ggplot() +
  geom_sf(data = GBR_sf) +
  geom_sf(data = pts_holes_tot,
          aes(fill = dist),
          colour = NA) +
  scale_fill_brewer(direction = 2)

Remove areas in the sea

If you want to find proximity area on terrestrial parts only, we need to remove buffer areas that are in the sea. Intersection is computed between multipolygons with the same projection. I previously realize an union of the UK map.

# Remove part of polygons in the sea
# Union and projection of UK map
GBR_sf_merc <- st_transform(st_union(GBR_sf), 27700)
pts_holes_uk <- st_intersection(pts_holes_tot, 
                              GBR_sf_merc)

ggplot() +
  geom_sf(data = GBR_sf) +
  geom_sf(data = pts_holes_uk,
          aes(fill = dist),
          colour = NA) +
  scale_fill_brewer(direction = 2)

And here is the final proximity map using sf , ggplot2 and a few other libraries...

Answer 2

Based on Sébastien's example, a more old-fashioned approach:

library(raster)
GBR <- getData(name = "GADM", country = "GBR", level = 1)
pts <- matrix(c(-0.4966766, -2.0772529, -3.8437793, 51.91829, 52.86147, 56.73899), ncol = 2)

r <- raster(GBR, res=1/12)
d <- distanceFromPoints(r, pts)
m <- mask(d, GBR)

plot(m)