R package for calculating pairwise distances on dual-weighted directed graphs using Priority Queue Shortest Paths. Dual-weighted directed graphs are directed graphs with two sets of weights so that weight1(A->B) != weight1(B->A)
---the directed property---and weight2(A->B) != weight1(A->B)
---the dual property. dodgr
calculates shortest paths according to one weight, while distances along paths are calculating using the other weight. A canonical example of a dual-weighted directed graph is a street network to be used for routing. Routes are usually calculated by weighting different kinds of streets or ways according to a particular mode of transport, while the desired output is a direct, unweighted distance.
The shortest path algorithm relies on priority queue code adapted from original code by Shane Saunders available here http://www.cosc.canterbury.ac.nz/tad.takaoka/alg/spalgs/spalgs.html
You can install dodgr
from github with:
# install.packages("devtools")
devtools::install_github("gmost/dodgr")
The primary function,
d <- dodgr_dists (graph = graph, from = pts, to = pts)
produces a square matrix of distances between all points listed in pts
and routed along the dual-weighted directed network given in graph
. An even simpler usage allows calculation of pair-wise distances between a set of geographical coordinates (here, for a sizey chunk of New York City):
xlim <- c (-74.12931, -73.99214)
ylim <- c (40.70347, 40.75354)
npts <- 1000
pts <- data.frame (x = xlim [1] + runif (npts) * diff (xlim),
y = ylim [1] + runif (npts) * diff (ylim))
st <- Sys.time ()
d <- dodgr_dists (from = pts)
Sys.time () - st
#> Time difference of 9.473597 secs
range (d, na.rm = TRUE)
#> [1] 0.00000 16.64285
This will automatically download the street network (using osmdata
), and even then calculating distances between 1,000 points can be done in around 10 seconds.
A graph or network in dodgr
is represented as a flat table (data.frame
, tibble
, data.table
, whatever) of minimally four columns: from
, to
, weight
, and distance
. The first two can be of arbitrary form (numeric
or character
); weight
is used to evaluate the shortest paths, and the desired distances are evaluated by summing the values of distance
along those paths. For a street network example, weight
will generally be the actual distance multiplied by a priority weighting for a given mode of transport and type of way, while distance
will be the pysical distance.
For more detail, see the main package vignette, along with a second vignette detailing benchmark timings, showing that under many circumstances, dodgr
performs considerably faster than equivalent routines from the igraph
package.