spNetwork (version 0.4.4.6)

calc_isochrones: Isochrones calculation

Description

Calculate isochrones on a network

Usage

calc_isochrones(
  lines,
  dists,
  start_points,
  donught = FALSE,
  mindist = 1,
  weight = NULL,
  direction = NULL
)

Value

A feature collection of lines representing the isochrones with the following columns

• point_id: the index of the point at the centre of the isochrone;

• distance: the size of the isochrone

Arguments

lines

A feature collection of lines representing the edges of the network

dists

A vector of the size of the desired isochrones. Can also be a list of vector when each start point must have its own distances. If so, the length of the list must be equal to the number of rows in start_points.

start_points

A feature collection of points representing the starting points if the isochrones

donught

A boolean indicating if the returned lines must overlap for each distance (FALSE, default) or if the lines must be cut between each distance step (TRUE).

mindist

The minimum distance between two points. When two points are too close, they might end up snapped at the same location on a line. Default is 1.

weight

The name of the column in lines to use an edge weight. If NULL, the geographical length is used. Note that if lines are split during the network creation, the weight column is recalculated proportionally to the new lines length.

direction

The name of the column indicating authorized travelling direction on lines. if NULL, then all lines can be used in both directions (undirected). The values of the column must be "FT" (From - To), "TF" (To - From) or "Both".

Details

An isochrone is the set of reachable lines around a node in a network within a specified distance (or time). This function perform dynamic segmentation to return the part of the edges reached and not only the fully covered edges. Several start points and several distances can be given. The network can also be directed. The lines returned by the function are the most accurate representation of the isochrones. However, if polygons are required for mapping, the vignette "Calculating isochrones" shows how to create smooth polygons from the returned sets of lines.

Examples

library(sf)
# creating a simple network
wkt_lines <- c(
  "LINESTRING (0.0 0.0, 5.0 0.0)",
  "LINESTRING (0.0 -5.0, 5.0 -5.0)",
  "LINESTRING (5.0 0.0, 5.0 5.0)",
  "LINESTRING (5.0 -5.0, 5.0 -10.0)",
  "LINESTRING (5.0 0.0, 5.0 -5.0)",
  "LINESTRING (5.0 0.0, 10.0 0.0)",
  "LINESTRING (5.0 -5.0, 10.0 -5.0)",
  "LINESTRING (10.0 0, 10.0 -5.0)",
  "LINESTRING (10.0 -10.0, 10.0 -5.0)",
  "LINESTRING (15.0 -5.0, 10.0 -5.0)",
  "LINESTRING (10.0 0.0, 15.0 0.0)",
  "LINESTRING (10.0 0.0, 10.0 5.0)")

linesdf <- data.frame(wkt = wkt_lines,
                      id = paste("l",1:length(wkt_lines),sep=""))

lines <- st_as_sf(linesdf, wkt = "wkt", crs = 32188)

# and the definition of the starting point
start_points <- data.frame(x=c(5),
                           y=c(-2.5))
start_points <- st_as_sf(start_points, coords = c("x","y"), crs = 32188)

# setting the directions

lines$direction <- "Both"
lines[6,"direction"] <- "TF"

isochrones <- calc_isochrones(lines,dists = c(10,12),
                              donught = TRUE,
                              start_points = start_points,
                              direction = "direction")