Build a routable network graph from a line layer

Builds the node-edge graph a shortest-path query needs: nodes at line endpoints, edges weighted by geometry length or a cost column. The graph is held resident in an external pointer and passed to spatial_route() and spatial_service_area(), which stream origin (and destination) batches past it. This is the network counterpart of a resident sf y in spatial_knn(): the graph is the resident budget (bounded by the network size), while the query side scales by streaming.

Usage

spatial_network(
  lines,
  weight = NULL,
  directed = FALSE,
  direction = NULL,
  weight_to = NULL,
  tolerance = 0,
  geom = "geometry",
  crs = NA
)

Arguments

lines: An sf or sfc object of (multi)linestrings, or a vectra_node carrying a hex-WKB line geometry column (it is materialised).
weight: Name of a column in lines holding each edge's traversal cost. NULL (default) uses the geometry length (sf::st_length()).
directed: If TRUE, edges are one-way (by direction, or the digitised from->to direction). FALSE (default) makes every edge two-way.
direction: Name of a column of one-way codes, used when directed: "B" two-way, "FT" from->to only, "TF" to->from only, "N" closed (the +/-/0 sign convention is also accepted). NULL uses "FT" for every line, or "B" when weight_to is given.
weight_to: Name of a column holding the reverse-direction cost on a two-way edge of a directed graph. NULL reuses weight.
tolerance: Endpoints within this distance (CRS units) are snapped to one node. 0 (default) joins only exactly-coincident endpoints.
geom, crs: Geometry column name and CRS, as in spatial_map(). The CRS defaults to the one lines carries.

Value

A vectra_network object: the resident graph (an external pointer plus the node coordinates, edge table, and source-line geometry needed to snap queries and rebuild routes). Print it to see the node, edge, and connected-component counts.

Details

Endpoints within tolerance of each other are snapped to a single node, so a layer whose touching lines do not share exactly-equal endpoint coordinates still connects. The input must be (multi)linestrings; a MULTILINESTRING is split into its parts, each becoming one edge with the source attributes. Lines are treated as already split at their junctions (the usual road-network convention); two lines that merely cross in their interiors are not connected unless they share an endpoint. The graph and the Dijkstra solver are native C; sf (an optional dependency, Suggests) supplies only the geometry.

Examples

# A small grid of streets.
mk <- function(x1, y1, x2, y2)
  sf::st_linestring(rbind(c(x1, y1), c(x2, y2)))
streets <- sf::st_sfc(
  mk(0, 0, 1, 0), mk(1, 0, 2, 0), mk(0, 0, 0, 1),
  mk(0, 1, 1, 1), mk(1, 0, 1, 1), mk(1, 1, 2, 1), mk(2, 0, 2, 1))
net <- spatial_network(streets)
net