equivalent of sp package's point in polygon / overing using sf

Question

I am migrating code from sp package to the newer sf package. My previous code I had a polygon SpatialDataFrame (censimentoMap) and a SpatialPointDataFrame (indirizzi.sp) and I got the polygon cell id ("Cell110") for each point laying within with the instruction below:

points.data <- over(indirizzi.sp, censimentoMap[,"Cell110"])

Actually I created two sf objects:

shape_sf <- st_read(dsn = shape_dsn) shape_sf <- st_transform(x=shape_sf, crs=crs_string) and indirizzi_sf = st_as_sf(df, coords = c("lng", "lat"), crs = crs_string)

And I am looking for the sf equivalent of the above instruction... Migth it be:

ids<-sapply(st_intersects(x=indirizzi_sf,y=shshape_sfpeCrif), function(z) if (length(z)==0) NA_integer_ else z[1]) cell_ids <- shape_sf[ids,"Cell110"]

Answer 1

You can get the same result by using st_join: First create a demo polygon and some points with sf.

library(sf)
library(magrittr)

poly <- st_as_sfc(c("POLYGON((0 0 , 0 1 , 1 1 , 1 0, 0 0))")) %>% 
  st_sf(ID = "poly1")    

pts <- st_as_sfc(c("POINT(0.5 0.5)",
                   "POINT(0.6 0.6)",
                   "POINT(3 3)")) %>%
  st_sf(ID = paste0("point", 1:3))

now see the result using over on sp objects

over(as(pts, "Spatial"), as(polys, "Spatial"))
>#      ID
># 1 poly1
># 2 poly1
># 3  <NA>

now equivalent with sf st_join

st_join(pts, poly, join = st_intersects)
># Simple feature collection with 3 features and 2 fields
># geometry type:  POINT
># dimension:      XY
># bbox:           xmin: 0.5 ymin: 0.5 xmax: 3 ymax: 3
># epsg (SRID):    NA
># proj4string:    NA
>#     ID.x  ID.y               .
># 1 point1 poly1 POINT (0.5 0.5)
># 2 point2 poly1 POINT (0.6 0.6)
># 3 point3  <NA>     POINT (3 3)

or for the exact same result

as.data.frame(st_join(pts, poly, join = st_intersects))[2] %>% setNames("ID")

>#    ID
># 1 poly1
># 2 poly1
># 3  <NA>

Answer 2

Instead of using

st_join(pts, poly, join = st_intersects)

you can just use

st_intersection(pts, poly)

It is faster, and gives only points that are inside the poly.