[R] gdistance::accCost fails when nrows is bigger than 90

[Deniz Uğur]

Hello all,

It has been only 2 days since I started using R and I was trying movecost library. Before explaining the unexpected behavior I should say that I’m calling R function from Python using rpy2 package. That shouldn’t be an issue because everything works as expected when a matrix of size 100x90 is given.

Okay now, basically I’m converting an 2D matrix to RasterLayer with appropriate CRS and extent properties then transfer it to R runtime. A modified version of Tobler’s hiking function is applied then a shortest-path is calculated. This procedure works for a matrix with column size anywhere between 1 and memory’s limit. However if row size of the matrix exceeds 90 then function is halted on gdistance::accCost saying that:

"At structural_properties.c:5313 : cannot run Bellman-Ford algorithm, Negative loop detected while calculating shortest paths”

It’s very interesting issue because I have explicitly tried …88, 89, 90, 91 and it halted on +90

I’ve created a gist in case you want to try the application on your computer. Also I’ve included a clean version of the R script I was using bellow.

Thank you.

https://gist.github.com/DenizUgur/01e18edd03321f3d3928c3afb2bd7145 <https://gist.github.com/DenizUgur/01e18edd03321f3d3928c3afb2bd7145>

movecost <- function (dtm, origin, destin, time="s") {

  altDiff <- function(x){x[2] - x[1]}
  hd <- gdistance::transition(dtm, altDiff, 8, symm=FALSE)

  slope <- gdistance::geoCorrection(hd)

  cost_function <- function(x){ ifelse(x[adj] > 0, 1.3 * exp(-3.5 * abs(x[adj] + 0.05)), 5.3 * exp(-3.5 * abs(x[adj] + 0.05))) }

  adj <- raster::adjacent(dtm, cells=1:ncell(dtm), pairs=TRUE, directions=8)
  speed <- slope
  speed[adj] <- cost_function(slope)
  speed <- speed * 0.278
  Conductance <- gdistance::geoCorrection(speed)

  accum_final <- gdistance::accCost(Conductance, sp::coordinates(origin))

  accum_final <- raster::mask(accum_final, dtm)

  sPath <- gdistance::shortestPath(Conductance, sp::coordinates(origin), sp::coordinates(destin), output="SpatialLines")
  sPath$length <- rgeos::gLength(sPath, byid=TRUE)
  destin$cost <- raster::extract(accum_final, destin)
  
  results <- list("accumulated.cost.raster"=accum_final,
                  "LCPs"=sPath,
                  "dest.loc.w.cost"=destin)
}
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