Calculating Driving Distances using sfnetworks

I am trying to calculate the driving distance (e.g. based on road networks) between two sets of coordinates.

For example:

CN Tower: 290 Bremner Blvd, Toronto, ON M5V 3L9 (-79.61203, 43.68312)
Toronto Airport: 6301 Silver Dart Dr, Mississauga, ON L5P 1B2 (-79.61203, 43.64256)

To solve this problem, I tried to download the shapefile for the Canadian Road Network and subset it for the Province of Ontario:

    library(sf)
    library(rgdal)
    library(sfnetworks)
    # Set the URL for the shapefile
    url <- "https://www12.statcan.gc.ca/census-recensement/2011/geo/RNF-FRR/files-fichiers/lrnf000r22a_e.zip"

    # Create a temporary folder to download and extract the shapefile
    temp_dir <- tempdir()
    temp_file <- file.path(temp_dir, "lrnf000r22a_e.zip")

    # Download the shapefile to the temporary folder
    download.file(url, temp_file)

    # Extract the shapefile from the downloaded zip file
    unzip(temp_file, exdir = temp_dir)

    # Read the shapefile  and subset to Ontario
    a = st_read(file.path(temp_dir, "lrnf000r22a_e.shp"), query="select * from lrnf000r22a_e where PRUID_R ='35'")

Then, by consulting different references (e.g. https://cran.r-project.org/web/packages/sfnetworks/vignettes/sfn01_structure.html) - I tried to calculate the distance between these two points:

   # convert the shapefile to an sfnetwork object
    net <- as_sfnetwork(a)

    # define your start and end points
    q1 <- st_point(c(-79.61203, 43.68312))
    q2 <- st_point(c(-79.38709, 43.64256))

    # set the CRS of the points to match the CRS of your shapefile
    q1 <- st_sfc(q1, crs = st_crs(a))
    q2 <- st_sfc(q2, crs = st_crs(a))

    # find the shortest path between the two points
    path <- st_network_paths(net, q1, q2)

    # calculate the distance of the path in meters
    distance <- sum(st_length(path))

But I get the following error:
Error in UseMethod("st_geometry") : no applicable method for 'st_geometry' applied to an object of class "c('tbl_df', 'tbl', 'data.frame')"

Can someone please show me how to fix this problem? Thanks!

Hi @swaheera,

After reviewing your code I see a couple of issues.

The way you define your start and end points is not entirely correct. When you set the CRS, you are basically telling sf to assign a EPSG:3347 to coordinates that actually are in EPSG:4326 as given above. To correct this you should do something like:

q1 <- st_transform(st_sfc(q1, crs = st_crs(4326)) , st_crs(a))
q2 <- st_transform(st_sfc(q2, crs = st_crs(4326)) , st_crs(a))

When you create the network as_sfnetwork() defaults to directed networks. This usually is a tricky setting knowing that many road networks are not correctly digitized. The algorithm doesnot find a path between your points because of the direction of the edges. To avoid this you can use:

net <- as_sfnetwork(a, directed = FALSE)

In your original code you were getting a result already stored on the paths object. This is where all the nodes and edges of you resulting path are located as list objects. If you are only looking for the distance however, I would recommend you to call st_network_cost(net, q1, q2). This will directly give you the shortest network distance in meters between your two points.

If hope you don't mind that I convert this issue to a discussion. If you have any question we can discuss there 😄

luukvdmeer / sfnetworks

Calculating Driving Distances using sfnetworks #240