Limit Simple Features Plot based on geojson Polygon - r

Question

I'd like to limit a plot based on a polygon defined in geojson, so that it only shows the area shaded blue here.
i.e. just plot the features inside and including the ring road.
The geojson is available here.
It would also be great to add a buffer around the edge to include the ring road.
My code to draw all the features (unlimited by the geojson is below).
library(tidyverse)
library(osmdata)
bounding_box <- getbb("Birmingham", featuretype = "city")
streets <- bounding_box %>%
opq()%>%
add_osm_feature(key = "highway",
value = c("motorway", "trunk", "primary", "secondary", "tertiary")) %>%
osmdata_sf()
ggplot() +
geom_sf(data = streets$osm_lines,
inherit.aes = FALSE,
color = "grey",
size = 1) +
theme_void() +
theme(
plot.background = element_rect(fill = "white"),
legend.position = "none"
) +
coord_sf(xlim = c(-1.933, -1.869),
ylim = c(52.46, 52.496),
expand = FALSE)

Answer 1

I assume in the following that the object streets has already been defined by running the first few lines of the code in the question. The next step is then to read the polygon using read_sf() from the sf package. The next line converts to a more suitable coordinate system (OSGB 1936 / British National Grid) because adding a buffer in meters is not possible in lon/lat-coordinates. A buffer of 40 meters is added using st_buffer() and finally the coordinates are transformed back to WGS84:
library(sf)
area <- read_sf("~/Birmingham CAZ 2020.GeoJSON") %>%
st_transform(27700) %>%
st_buffer(units::set_units(40, m)) %>%
st_transform(4326)
Of course, you need to adapt the path to where you have actually stored the file. Then I use st_intersection() to extract the part of streets$osm_lines that lies inside the polygon:
streets_area <- st_intersection(poly, streets$osm_lines)
And finally I produce the plot using the code from your question. Note that I have added a layer with the polygon in the second line in order to demonstrate that the streets indeed lie inside the polygon:
ggplot() +
geom_sf(data = area) +
geom_sf(data = streets_area,
inherit.aes = FALSE,
color = "grey",
size = 1) +
theme_void() +
theme(
plot.background = element_rect(fill = "white"),
legend.position = "none"
) +
coord_sf(xlim = c(-1.933, -1.869),
ylim = c(52.46, 52.496),
expand = FALSE)