12.4 Grid of points

df_sf <- df %>%
  st_as_sf(coords = c("longitude","latitude"),
           # or use crs = 4326
           crs = "+proj=longlat +datum=WGS84") %>%
  st_intersection(ctr_africa) %>%
  st_make_valid()

grid <- df_sf %>%
  st_bbox() %>%
  st_as_sfc() %>%
  st_make_grid(what = "centers",
               cellsize = .2,
               square = F)