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I want to make a data frame showing which voting precincts are in each school zone. To do this, I want to see which precincts' center points are in which school zones.

library(sf)

precincts <- st_read("voting_precincts.shp")
precincts$centroid <- st_centroid(
  x = precincts
)

elemZones <- read_sf("school_zones.shp")

So although I have the centroid coordinates for precincts, I do not know how to "join" them with the school zones data.

1 Answer 1

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One of the few limitations simple feature standard for geometries has is that it cannot have two geometry columns; you may have two or more columns with WKT definitions but you have to decide which column to use. In your case you try to add the centroid as a column, but what sf does is to add a the whole sf tbl object (the data frame) as a column, if you print the object with str(precincts) you'll see the classes of each column, if you just print str(precincts$centroid) you'll get:

 str(precincts$centroid)
 Classes ‘sf’, ‘tbl_df’, ‘tbl’ and 'data.frame'

st_centroid will return a whole sf tbl, so you may just do:

 library(sf)
 library(dplyr)
 precincts <- st_read("voting_precincts.shp")
 elemZones <- read_sf("school_zones.shp")

 st_centroid(precincts) %>% st_join(elemZones)
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  • Worked great. Side note I noticed I used st_read() and read_sf(). That was by mistake. But is there a difference between the two functions?
    – Username
    Commented Nov 16, 2020 at 21:22
  • 2
    read_sf and write_sf are aliases for st_read and st_write, respectively, with some modified default arguments. read_sf and write_sf are quiet by default: they do not print information about the data source. read_sf returns an sf-tibble rather than an sf-data.frame. write_sf delete layers by default: it overwrites existing files without asking or warning. (from the sf package reference)
    – Elio Diaz
    Commented Nov 16, 2020 at 22:22

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