Skip to contents

Compute isolines around nodes in a spatial network

Source: R/iso.R
st_network_iso.Rd

Isolines are curves along which a function has a constant value. In spatial networks, they are used to delineate areas that are reachable from a given node within a given travel cost. If the travel cost is distance, they are known as isodistances, while if the travel cost is time, they are known as isochrones. This function finds all network nodes that lie inside an isoline around a specified node.

Usage

st_network_iso(
  x,
  node,
  cost,
  weights = edge_length(),
  ...,
  delineate = TRUE,
  ratio = 1,
  allow_holes = FALSE
)

Arguments

x

An object of class sfnetwork.

node

The node around which the isolines will be drawn. Evaluated by evaluate_node_query. When multiple nodes are given, only the first one is used.

cost

The constant cost value of the isoline. Should be a numeric value in the same units as the given edge weights. Alternatively, units can be specified explicitly by providing a units object. Multiple values may be given, which will result in multiple isolines being drawn.

weights

The edge weights to be used in the shortest path calculation. Evaluated by evaluate_weight_spec. The default is edge_length, which computes the geographic lengths of the edges.

...

Additional arguments passed on to st_network_cost to compute the cost matrix from the specified node to all other nodes in the network.

delineate

Should the nodes inside the isoline be delineated? If FALSE, the nodes inside the isoline are returned as a MULTIPOINT geometry. If TRUE, the concave hull of that geometry is returned instead. Defaults to TRUE.

ratio

The ratio of the concave hull. Defaults to 1, meaning that the convex hull is computed. See st_concave_hull for details. Ignored if delineate = FALSE. Setting this to a value smaller than 1 requires a GEOS version of at least 3.11.

allow_holes

May the concave hull have holes? Defaults to FALSE. Ignored if delineate = FALSE.

Value

An object of class sf with one row per requested isoline. The object contains the following columns:

  • cost: The constant cost value of the isoline.

  • geometry: If delineate = TRUE, the concave hull of all nodes that lie inside the isoline. Otherwise, those nodes combined into a single MULTIPOINT geometry.

Examples

library(sf, quietly = TRUE)

oldpar = par(no.readonly = TRUE)
par(mar = c(1,1,1,1))

center = st_centroid(st_combine(st_geometry(roxel)))

net = as_sfnetwork(roxel, directed = FALSE)

iso = net |>
  st_network_iso(node_is_nearest(center), c(1000, 500, 250))

colors = c("#fee6ce90", "#fdae6b90", "#e6550d90")

plot(net)
plot(st_geometry(iso), col = colors, add = TRUE)


# The level of detail can be increased with the ratio argument.
# This requires GEOS >= 3.11.
if (compareVersion(sf_extSoftVersion()[["GEOS"]], "3.11.0") > -1) {

  iso = net |>
    st_network_iso(node_is_nearest(center), c(1000, 500, 250), ratio = 0.3)

  colors = c("#fee6ce90", "#fdae6b90", "#e6550d90")

  plot(net)
  plot(st_geometry(iso), col = colors, add = TRUE)
}

par(oldpar)