R: gdistance different results for accCost and costDistance

Question

accCost() and costDistance() functions from R gdistance produce different values when going from source coordinate A to destination coordinate B. Shouldn't the cost accumulation value at B be equivalent to the costDistance value from A to B given an equivalent anisotropic transition matrix and that both functions use the Dijkstra algorithm?

If not, then what is the fundamental difference between the calculations? If so, what accounts for the different values derived from the code presented below? In the example, A to B costDistance=0.13 hours and accCost=0.11 hours at point B. My other tests suggest that accCost is consistently less than costDistance and consierably so over long distances. The code is based on the example provided in accCost documentation.

require(gdistance)
r <- raster(system.file("external/maungawhau.grd", package="gdistance"))
altDiff <- function(x){x[2] - x[1]}
hd <- transition(r, altDiff, 8, symm=FALSE)
slope <- geoCorrection(hd)
adj <- adjacent(r, cells=1:ncell(r), pairs=TRUE, directions=8)
speed <- slope
speed[adj] <- 6 * 1000 * exp(-3.5 * abs(slope[adj] + 0.05))#1000 to convert to a common spatial unit of meters
Conductance <- geoCorrection(speed)
A <- matrix(c(2667670, 6479000),ncol=2)
B <- matrix(c(2667800, 6479400),ncol=2)
ca <- accCost(Conductance,fromCoords=A)
extract(ca,B)
costDistance(Conductance,fromCoords=A,toCoords=B)

Please provide a reproducible example. http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example — Michael Veale, Apr 10 '16 at 09:55
I have added the line, "require(gdistance)", which should make the example fully reproducible. — RandyHaas, Apr 10 '16 at 21:28
@RandyHaas did you had any errors like "Bellman-Ford detected negative cycles" when running accCost? — Deniz da King, May 13 '20 at 22:17

JacobVanEtten · Accepted Answer · 2016-04-11T22:36:41.273

There should be no difference. The current version of accCost has a small bug that arises from a change in the igraph package.

For the moment, please see if this function solves the problem.

setMethod("accCost", signature(x = "TransitionLayer", fromCoords = "Coords"),
 def = function(x, fromCoords)
  {
    fromCoords <- .coordsToMatrix(fromCoords) 
    fromCells <- cellFromXY(x, fromCoords)
    if(!all(!is.na(fromCells))){
        warning("some coordinates not found and omitted")
        fromCells <- fromCells[!is.na(fromCells)]
    }
    tr <- transitionMatrix(x)
    tr <- rBind(tr,rep(0,nrow(tr)))
    tr <- cBind(tr,rep(0,nrow(tr)))

    startNode <- nrow(tr) #extra node to serve as origin
    adjP <- cbind(rep(startNode, times=length(fromCells)), fromCells)

    tr[adjP] <- Inf

    adjacencyGraph <- graph.adjacency(tr, mode="directed", weighted=TRUE)
    E(adjacencyGraph)$weight <- 1/E(adjacencyGraph)$weight      

    shortestPaths <- shortest.paths(adjacencyGraph, v=startNode, mode="out")[-startNode]

    result <- as(x, "RasterLayer")
    result <- setValues(result, shortestPaths)  
    return(result)
  }
)

Thank you. I tried this and get the following error: Error in accCost(Conductance, fromCoords = A) : could not find function ".coordsToMatrix" — RandyHaas, Apr 13 '16 at 21:19
Sorry, this will work only if you get that function from here: https://r-forge.r-project.org/scm/viewvc.php/pkg/gdistance/R/internal-functions.R?view=markup&revision=222&root=gdistance This is a temporary solution only and should be resolved soon. — JacobVanEtten, Apr 14 '16 at 02:53

score 0 · Answer 2 · answered Feb 28 '17 at 14:38

0

This issue has been resolved in gdistance 1.2-1.

answered Feb 28 '17 at 14:38

JacobVanEtten

203
1
7

R: gdistance different results for accCost and costDistance

2 Answers2