From base to ggplot2: Converting the NC affine transformation example from sf vignette #3

sf3_base_to_ggplot2.R

# The following is a "conversion" from base to ggplot2
# of the affine transformation from the sf package's
# third vignette:
# https://cran.r-project.org/web/packages/sf/vignettes/sf3.html#affine-transformations

library(sf)
library(magrittr)

nc <- st_read(system.file("shape/nc.shp", package="sf"), quiet = TRUE)

cntrd <- nc %>%
  st_geometry() %>% # not needed
  st_centroid

rot <- function(a) matrix(c(cos(a), sin(a), -sin(a), cos(a)), 2, 2)

ncg2 <- (nc %>% st_geometry() - cntrd) * rot(pi / 2) * 0.75 + cntrd

# ggplot2::geom_sf() only plays with objects of class sf.
# Therefore, two more sf objects are needed in order to
# assign the above class sfc objects into.

## ----
# UPDATE: not necessary. Can supply class sfc objects
# directly to data argument in geom_sf().
# See: https://github.com/tidyverse/ggplot2/issues/2073
## ----

nc2 <- nc
nc3 <- nc

st_geometry(nc2) <- ncg2
st_geometry(nc3) <- cntrd

# ALSO: nc2 is now missing its epsg and proj4string.
# This must be assigned before plotting.

st_crs(nc2) <- st_crs(nc3)

# Now we may plot.

library(ggplot2)  # devtools::install_github("tidyverse/ggplot2")
library(ggthemes)
nc %>%
  ggplot() +
  geom_sf(color = "gray", fill = "white") +
  geom_sf(data = nc2, color = "black", fill = "white") +
  geom_sf(data = nc3, color = "red", shape = 1, size = 0.25) +
  theme_map()

# The only problem with this "conversion" is that
# the red circles are now very squiggly circles.