Compute isochrones (service areas)

For one or more origin nodes, finds the set of nodes reachable within each cutoff (by the chosen edge weight — distance or, with ox_add_edge_travel_times(), travel time) and returns a polygon per cutoff: the hull of the reachable nodes. With several origins, reachability is the minimum cost from any origin.

Usage

ox_isochrone(g, center, cutoffs, weight = "length", ratio = 0.4)

Arguments

g

An osm_graph.

center

One or more origin node osmids (see ox_nearest_nodes()).

cutoffs

Numeric vector of cutoff values, in the units of weight.

weight

Edge column used as cost. Default "length".

ratio

Concavity for sf::st_concave_hull() (0 = most concave, 1 = convex). Default 0.4.

Value

An sf object with one polygon row per cutoff (columns cutoff, n_nodes, geometry), ordered from largest to smallest cutoff so smaller areas draw on top.

Details

Reachable sets come from the Rust Dijkstra core; the hull is built with sf::st_concave_hull() when available (GEOS >= 3.11), falling back to a convex hull. For metric cutoffs, project the graph to a metric CRS first.

Examples

g <- example_osm_graph(n = 6, spacing = 100)
center <- ox_nearest_nodes(g, 250, 250)
iso <- ox_isochrone(g, center, cutoffs = c(100, 300))
iso
#> Simple feature collection with 2 features and 2 fields
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: 0 ymin: 0 xmax: 500 ymax: 500
#> Projected CRS: WGS 84 / Pseudo-Mercator
#>   cutoff n_nodes                       geometry
#> 1    300      23 POLYGON ((0 200, 0 300, 100...
#> 2    100       5 POLYGON ((200 200, 100 200,...