How can I crop the data points which lie inside polygon and plot

Question

How can I crop the data points which lie inside the given polygon?

library(tidyverse)

tbl <- tibble(x = runif(100),
              y = runif(100))

ggplot(data = tbl,
       aes(x = x,
           y = y)) + 
  geom_point() + 
  theme_bw() + 
  coord_equal()

poly <- tibble(A = c(0.5, 0),
               B = c(1, 0.5),
               C = c(0.5, 1),
               D = c(0, 0.5))

Answer 1

One way could be to convert the points and polygon to sf objects, crop the points, then turn the data back into a tibble.

library(sf)
library(tidyverse)
library(gridExtra)

set.seed(345)

tbl <- tibble(x = runif(100),
              y = runif(100))

poly <- tibble(
  A = c(0.5, 0),
  B = c(1, 0.5),
  C = c(0.5, 1),
  D = c(0, 0.5)
)

# Convert tbl to sf object.
tbl_sf <- sf::st_as_sf(tbl, coords = c("x", "y"))

# Convert polygon to sf polygon.
poly_sf <- as.data.frame(t(poly)) %>%
  dplyr::rename(x = V1, y = V2) %>%
  sf::st_as_sf(coords = c("x", "y")) %>%
  dplyr::summarise() %>%
  sf::st_cast("POLYGON") %>%
  sf::st_convex_hull()

# Keep only points inside the polygon.
points <- sf::st_intersection(tbl_sf, poly_sf) %>%
  
# Get coordinates and convert back to tibble.
  sf::st_coordinates() %>%
  as_tibble() %>%
  dplyr::rename(x = X, y = Y)

before <- ggplot(data = tbl,
                 aes(x = x,
                     y = y)) +
  geom_point() +
  theme_bw() +
  coord_equal() +
  ggtitle("Before")

after <- ggplot(data = points,
                aes(x = x,
                    y = y)) +
  geom_point() +
  theme_bw() +
  coord_equal() +
  ggtitle("After")

grid.arrange(before, after, ncol = 2)

Output