Assuming I have two sf objects with the same coordinate reference system and non-overlapping area, how to correctly combine these two sf objects to create one sf objects?
Here is an example.
# Load packages
library(tidyverse)
library(sf)
# Load the example sf object
nc <- st_read(system.file("shape/nc.shp", package="sf"))
# print the first three rows of nc in the console
head(nc, 3)
# Simple feature collection with 3 features and 14 fields
# geometry type: MULTIPOLYGON
# dimension: XY
# bbox: xmin: -81.74107 ymin: 36.23388 xmax: -80.43531 ymax: 36.58965
# epsg (SRID): 4267
# proj4string: +proj=longlat +datum=NAD27 +no_defs
# AREA PERIMETER CNTY_ CNTY_ID NAME FIPS FIPSNO CRESS_ID BIR74 SID74 NWBIR74 BIR79 SID79 NWBIR79 geometry
# 1 0.114 1.442 1825 1825 Ashe 37009 37009 5 1091 1 10 1364 0 19 MULTIPOLYGON (((-81.47276 3...
# 2 0.061 1.231 1827 1827 Alleghany 37005 37005 3 487 0 10 542 3 12 MULTIPOLYGON (((-81.23989 3...
# 3 0.143 1.630 1828 1828 Surry 37171 37171 86 3188 5 208 3616 6 260 MULTIPOLYGON (((-80.45634 3...
The nc is an sf object and also a data frame. Most of the functions from the dplyr or tidyverse work well on the sf object. Below, I used the slice function to split the nc object to nc1 and nc2.
# Split the nc object to two sf objects
nc1 <- nc %>% slice(1:50)
nc2 <- nc %>% slice(51:100)
# Print the crs of nc, nc1, and nc2
st_crs(nc)
# Coordinate Reference System:
# EPSG: 4267
# proj4string: "+proj=longlat +datum=NAD27 +no_defs"
st_crs(nc1)
# Coordinate Reference System:
# EPSG: 4267
# proj4string: "+proj=longlat +datum=NAD27 +no_defs"
st_crs(nc2)
# Coordinate Reference System:
# EPSG: 4267
# proj4string: "+proj=longlat +datum=NAD27 +no_defs"
As you can see, nc, nc1, and nc2 all have the same coordinate-reference system, and there are no overlapping area between nc1 and nc2.
Since sf objects are data frames, my first thought to combine two data frames is to use bind_rows from the dplyr package. Howver, bind_rows gave me an warning.
# Combine nc1 and nc2 with bind_rows
nc_combine <- bind_rows(nc1, nc2)
# Warning messages:
# 1: In bind_rows_(x, .id) :
# Vectorizing 'sfc_MULTIPOLYGON' elements may not preserve their attributes
# 2: In bind_rows_(x, .id) :
# Vectorizing 'sfc_MULTIPOLYGON' elements may not preserve their attributes
The new object, nc_combine, is still an sf object and a data frame with the same row and column number as nc. However, I cannnot access any information in nc_combine, and the coordinate-reference information seems to be gone.
class(nc_combine)
# [1] "sf" "data.frame"
dim(nc_combine)
# [1] 100 15
nc_combine
# Error in .subset2(x, i, exact = exact) :
# attempt to select less than one element in get1index
In other words, my question is, how to combine nc1 and nc2 to re-create the nc object?