sf and networks in R : create an edgelist from distinct linestrings and points sf objects

Question

I've got an sf object of nodes (stops) and an other of linestrings (routes).

# toy example 

## nodes
p1 = st_point(c(7, 51))
p2 = st_point(c(7, 52))
p3 = st_point(c(7, 53))
p4 = st_point(c(8, 52))
nodes = st_as_sf(st_sfc(p1, p2, p3,p4, crs = 4326))



## routes

e1 = st_cast(st_union(p1,p3), "LINESTRING")
e2 = st_cast(st_union(p1,p4), "LINESTRING")
e3 = st_cast(st_union(p3,p4), "LINESTRING")
lines = st_as_sf(st_sfc(e1, e2, e3, crs = 4326))

how to get an edgelist of route sections that directly connect two nodes?

#Desired output

from | to
p1   | p2
p2   | p3
p1   | p4
p3   | p4

Here there is a line between p1 and p3 but p2 is in the middle so there are 2 edges between p1-p2 and p2-p3

I know that the sfnetwork package can construct network with linestrings or with points, but how can I create a spatial network with the intersection of lines and points ?

Answer 1

Not sure of what you are looking for but I give it another try as a follow-up to your comment!! So, please find below a fully edited reprex using sf, sfnetworks and dplyr libraries.

Reprex

• STEP 1: Create a network with your points/nodes p1, p3 and p4

library(sf)
library(sfnetworks)
library(dplyr)

# toy example 
p1 = st_point(c(7, 51))
p3 = st_point(c(7, 53))
p4 = st_point(c(8, 52))
nodes = st_as_sf(st_sfc(p1, p3, p4, crs = 4326))
nodes$names <- c("p1", "p3", "p4") # add the name of nodes


## routes
e1 = st_cast(st_union(p1,p3), "LINESTRING")
e2 = st_cast(st_union(p1,p4), "LINESTRING")
e3 = st_cast(st_union(p3,p4), "LINESTRING")
lines = st_as_sf(st_sfc(e1, e2, e3, crs = 4326))
lines$from  <-  c("p1", "p1", "p3")
lines$to  <-  c("p3", "p4", "p4")


# Create the network
network <- sfnetwork(nodes, lines, node_key = "names")
#> Checking if spatial network structure is valid...
#> Spatial network structure is valid


# What the network looks like:
network
#> # A sfnetwork with 3 nodes and 3 edges
#> #
#> # CRS:  EPSG:4326 
#> #
#> # A directed acyclic simple graph with 1 component with spatially explicit edges
#> #
#> # Node Data:     3 x 2 (active)
#> # Geometry type: POINT
#> # Dimension:     XY
#> # Bounding box:  xmin: 7 ymin: 51 xmax: 8 ymax: 53
#>             x names
#>   <POINT [°]> <chr>
#> 1      (7 51) p1   
#> 2      (7 53) p3   
#> 3      (8 52) p4   
#> #
#> # Edge Data:     3 x 3
#> # Geometry type: LINESTRING
#> # Dimension:     XY
#> # Bounding box:  xmin: 7 ymin: 51 xmax: 8 ymax: 53
#>    from    to                x
#>   <int> <int> <LINESTRING [°]>
#> 1     1     2     (7 51, 7 53)
#> 2     1     3     (7 51, 8 52)
#> 3     2     3     (7 53, 8 52)

• STEP 2: Add a supplementary node (i.e. p2) to the 'network' object

# Create a 'sf' object for point 2 'p2'
p2 = st_point(c(7, 52))
p2_sf <- st_as_sf(st_sfc(p2, crs = 4326))
p2_sf$names <- "p2"

# Add 'p2' to the network and get the 'new_network' object
new_network <- st_network_blend(network, p2_sf)


# Cleaning the 'new_network' object
new_network %>% 
  activate("nodes") %>% 
  mutate(names = coalesce(names.x, names.y)) %>% 
  arrange(., names) %>%
  select(names,x) -> new_network

• STEP 3: Create an sf object with the desired result

new_network %>% 
  activate("edges") %>% 
  st_as_sf() %>% 
  mutate(from = paste0("p", from), to = paste0("p", to))

#> Simple feature collection with 4 features and 2 fields
#> Geometry type: LINESTRING
#> Dimension:     XY
#> Bounding box:  xmin: 7 ymin: 51 xmax: 8 ymax: 53
#> Geodetic CRS:  WGS 84
#> # A tibble: 4 x 3
#>   from  to                   x
#> * <chr> <chr> <LINESTRING [°]>
#> 1 p1    p2        (7 51, 7 52)
#> 2 p2    p3        (7 52, 7 53)
#> 3 p1    p4        (7 51, 8 52)
#> 4 p3    p4        (7 53, 8 52)

^{Created on 2021-12-05 by the reprex package (v2.0.1)}