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@internaut internaut/zoom.R
Created Apr 30, 2019

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Zooming in on maps with sf and ggplot2
# Source for blog post "Zooming in on maps with sf and ggplot2"
# URL: https://datascience.blog.wzb.eu/2019/04/30/zooming-in-on-maps-with-sf-and-ggplot2/
#
# Markus Konrad <markus.konrad@wzb.eu>
# Wissenschaftszentrum Berlin für Sozialforschung
# April 30, 2019
#
#### world map ####
library(ggplot2)
library(sf)
library(rnaturalearth)
worldmap <- ne_countries(scale = 'medium', type = 'map_units', returnclass = 'sf')
head(worldmap[c('name', 'continent')])
ggplot() + geom_sf(data = worldmap) + theme_bw()
ggsave('zoom1_world.png')
#### select france ####
france <- worldmap[worldmap$name == 'France',]
ggplot() + geom_sf(data = france) + theme_bw()
ggsave('zoom2_france.png')
#### select europe ####
europe <- worldmap[worldmap$continent == 'Europe',]
ggplot() + geom_sf(data = europe) + theme_bw()
ggsave('zoom3_europe.png')
#### crop europe ####
europe_cropped <- st_crop(worldmap, xmin = -20, xmax = 45, ymin = 30, ymax = 73)
ggplot() + geom_sf(data = europe_cropped) + theme_bw()
ggsave('zoom4_europe.png')
#### crop europe -- variant: remove "gaps" / padding ####
ggplot() + geom_sf(data = europe_cropped) + coord_sf(expand = FALSE) + theme_bw()
#### crop europe -- variant: apply mollweide projection ####
ggplot() + geom_sf(data = europe_cropped) + coord_sf(crs = st_crs('+proj=moll')) + theme_bw()
ggsave('zoom4_europe_moll.png')
#### restrict display window on europe ####
ggplot() + geom_sf(data = worldmap) +
coord_sf(xlim = c(-20, 45), ylim = c(30, 73), expand = FALSE) +
theme_bw()
ggsave('zoom5_europe.png')
#### restrict display window on europe: fail ####
# this fails because xlim and ylim must be specified in the CRS that is used for projection (here: Mollweide)
ggplot() + geom_sf(data = worldmap) +
coord_sf(crs = st_crs('+proj=moll'), xlim = c(-20, 45), ylim = c(30, 73), expand = FALSE) +
theme_bw()
ggsave('zoom5_europe_fail.png')
#### restrict display window on europe using a different projection -- overview ####
target_crs <- '+proj=moll'
worldmap_trans <- st_transform(worldmap, crs = target_crs)
# display window corner points in the following order:
# top left, top right, bottom right, bottom left, top left (again, to close the shape)
# "crs = 4326" means that coordinates are supplied as WGS84 longitude/latitude coordinates
disp_win <- st_sfc(st_point(c(-20, 73)), st_point(c(45, 73)),
st_point(c(45, 30)), st_point(c(-20, 30)),
st_point(c(-20, 73)), crs = 4326)
disp_win
disp_win_trans <- st_transform(disp_win, crs = target_crs)
disp_win_lines <- st_sfc(st_linestring(st_coordinates(disp_win_trans)), crs = target_crs)
# corner points at bottom left and top right
# supply as WGS84 and directly transform to target CRS (Mollweide)
cornerpts <- data.frame(label = c('A', 'B'))
cornerpts$geometry <- st_transform(st_sfc(st_point(c(-20, 30)), st_point(c(45, 73)),
crs = 4326),
crs = target_crs)
cornerpts <- st_as_sf(cornerpts)
ggplot() + geom_sf(data = worldmap_trans) +
geom_sf(data = disp_win_lines, color = 'red') +
geom_sf_label(aes(label = label), data = cornerpts, color = 'red') +
coord_sf(datum = target_crs) + theme_bw() + theme(axis.title = element_blank())
ggsave('zoom6_europe_moll_dispwin.png')
#### restrict display window on europe using a different projection ####
target_crs <- '+proj=moll'
worldmap_trans <- st_transform(worldmap, crs = target_crs)
disp_win_wgs84 <- st_sfc(st_point(c(-20, 30)), st_point(c(45, 73)), crs = 4326)
disp_win_wgs84
disp_win_trans <- st_transform(disp_win_wgs84, crs = target_crs)
disp_win_trans
disp_win_coord <- st_coordinates(disp_win_trans)
ggplot() + geom_sf(data = worldmap_trans) +
coord_sf(xlim = disp_win_coord[,'X'], ylim = disp_win_coord[,'Y'], datum = target_crs, expand = FALSE) +
theme_bw()
ggsave('zoom6_europe_moll.png')
#### zoom to berlin ####
zoom_to <- c(13.38, 52.52) # Berlin
zoom_level <- 3
lon_span <- 360 / 2^zoom_level
lat_span <- 180 / 2^zoom_level
lon_bounds <- c(zoom_to[1] - lon_span / 2, zoom_to[1] + lon_span / 2)
lat_bounds <- c(zoom_to[2] - lat_span / 2, zoom_to[2] + lat_span / 2)
ggplot() + geom_sf(data = worldmap) +
geom_sf(data = st_sfc(st_point(zoom_to), crs = 4326), color = 'red') +
coord_sf(xlim = lon_bounds, ylim = lat_bounds) + theme_bw()
ggsave('zoom7_berlin.png')
#### zoom to kamchatka, different projection ####
zoom_to <- c(159.21, 56.41) # ~ center of Kamchatka
zoom_level <- 2.5
# Lambert azimuthal equal-area projection around center of interest
target_crs <- sprintf('+proj=laea +lon_0=%f +lat_0=%f', zoom_to[1], zoom_to[2])
C <- 40075016.686 # ~ circumference of Earth in meters
x_span <- C / 2^zoom_level
y_span <- C / 2^(zoom_level+1) # also sets aspect ratio
zoom_to_xy <- st_transform(st_sfc(st_point(zoom_to), crs = 4326), crs = target_crs)
zoom_to_xy
disp_window <- st_sfc(st_point(st_coordinates(zoom_to_xy - c(x_span / 2, y_span / 2))),
st_point(st_coordinates(zoom_to_xy + c(x_span / 2, y_span / 2))),
crs = target_crs)
ggplot() + geom_sf(data = worldmap) +
geom_sf(data = zoom_to_xy, color = 'red') +
coord_sf(xlim = st_coordinates(disp_window)[,'X'],
ylim = st_coordinates(disp_window)[,'Y'],
crs = target_crs,
datum = target_crs) +
theme_bw()
ggsave('zoom8_kamchatka.png')
# or, using the default graticule:
ggplot() + geom_sf(data = worldmap) +
geom_sf(data = zoom_to_xy, color = 'red') +
coord_sf(xlim = st_coordinates(disp_window)[,'X'],
ylim = st_coordinates(disp_window)[,'Y'],
crs = target_crs) +
theme_bw()
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