This is an update of my previous similar question, the same task only I need to do it within sf
framework.
I need to identify the inner boundaries between polygons, red lines in this map.
Within sp
framework I used to utilize a self written function that wrapped @Spacedman's answer. Here it is:
identify_borders <- function(SPolyDF){
require(rgeos)
require(sp)
borders <- gDifference(
as(SPolyDF,"SpatialLines"),
as(gUnaryUnion(SPolyDF),"SpatialLines"),
byid=TRUE)
df <- data.frame(len = sapply(1:length(borders),
function(i) gLength(borders[i, ])))
rownames(df) <- sapply(1:length(borders),
function(i) borders@lines[[i]]@ID)
SLDF <- SpatialLinesDataFrame(borders, data = df)
return(SLDF)
}
Alternatively one may use raster::boundaries()
.
# dev version of ggplot2 for geom_sf()
devtools::install_github("tidyverse/ggplot2")
library(tidyverse)
library(sf)
load(url("https://ikashnitsky.github.io/share/1712-so-q-identify-borders/geodata.Rdata"))
ggplot() +
geom_sf(data = gd_nuts0) +
geom_sf(data = gd_borders, color = "red") +
coord_sf(datum = NA) +
theme_void()
rmapshaper uses javascript, that is cheating! I tried:
i = st_intersection(gd_nuts0, gd_nuts0)
i2 <- i[i$nuts_id != i$nuts_id.1,]
plot(gd_nuts0[1])
plot(i2, add = TRUE, col ='red', lwd = 2)
It turned out that rmapshaper
has precisely the desired function and works with sf
objects nicely -- ms_innerlines()
. The only difficulty (possibly a bug) is that ms_innerlines()
returns a list rather than an sf
object. But this odd behavior is easily fixed. Below is the solution code. Note that I simplify the initial polygons to see the difference. The new inner borders that are created from an sf
object are plotted in navy color.
# dev version of ggplot2 for geom_sf()
devtools::install_github("tidyverse/ggplot2")
library(tidyverse)
library(sf)
library(rmapshaper)
load(url("https://ikashnitsky.github.io/misc/171211-so-question-identify-borders/geodata.Rdata"))
sf_poly_simp <- gd_nuts0 %>%
ms_simplify(keep = .2, keep_shapes = TRUE)
sf_bord_simp <- sf_poly_simp %>%
ms_innerlines() %>%
as_tibble() %>%
st_as_sf()
ggplot() +
geom_sf(data = sf_poly_simp) +
geom_sf(data = sf_bord_simp, color = "navy", size = 1) +
geom_sf(data = gd_borders, color = "red", size = .1) +
coord_sf(datum = NA) +
theme_void()
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