Converting a data.frame into a (MULTIPOLYGON) sf object

df_to_sf.R

library(sf)
library(dplyr)

# A function that closes the polygons if
# the first row of coordinates is different than
# the last rows of coordinates
add_first_row <- function(x) {
  if (!identical(
    as.numeric(x[1,]),
    as.numeric(x[nrow(x),])
    )
  ) {
    res <- rbind(x, x[1,])
  } else {
    res <- x
  }
  
  res
}

# Converts polygon coordinates into 
# simple features
coords_to_poly <- function(coords) {
  coords %>% 
    as.matrix %>% 
    list %>% 
    st_polygon %>% 
    st_sfc
}

# Test set
pdf <- structure(
  list(
    x = c(0L, 0L, 1L, 1L, 1L, 1L, 2L, 2L), 
    y = c(0L, 1L, 1L, 0L, 0L, 1L, 1L, 0L), 
    id = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L)
  ), 
  .Names = c("x", "y", "id"), 
  class = "data.frame", 
  row.names = c(NA, -8L)
)

pdf %>% 
  # id gives the polgon identifier in the original data.frame
  group_by(id) %>% 
  # check the polygons are closed
  add_first_row %>% 
  # create the geometry column
  summarise(
    geom = coords_to_poly(.)
  ) %>% 
  # converts to sf
  st_sf(crs = 4326) 

This seems to work

tst = split(pdf, pdf$id)

pol = do.call(c, lapply(seq(tst), function(i) {
  tmp = coords_to_poly(add_first_row(tst[[i]]))
  st_crs(tmp) = 4326
  return(tmp)
}))

Note, that you will still need to take care of the polygon dimensions, as for now they are "XYZ" where it seems that the id is used as the "Z" value...

@tim-salabim yes it is XYZ because I would need to explicitely pass the xy coordanmes in coords_to_poly