[R-sig-Geo] gdistance -- How to combine transition Layers?

[Thomas Chadefaux]

Great, thank you very much for your solution, Jacob. While I was
waiting, I tried fiddling around with the following, but nothing
really good came out of it. Your solution works well.

#failed solution:
tr.max <- tr.roads
tr.alt.num <- as.numeric(tr.alt[1:nrow(tr.roads),1:ncol(tr.roads)])
tr.roads.num <- as.numeric(tr.roads[1:nrow(tr.roads),1:ncol(tr.roads)])
max.speed <-  pmax(tr.alt.num, tr.roads.num)
max.speed <- as.integer(max.speed)   ## needs to be integer or next
step won't work
tr.max at transitionMatrix@p <- max.speed

Thanks again,
Thomas

On Mon, Sep 26, 2011 at 9:34 PM, Jacob van Etten
<jacobvanetten at yahoo.com> wrote:
> Hi Thomas,
>
> Currently, max() is not implemented in gdistance, because it is not
> implemented in Matrix.
>
> But there are ways to work around these limitations.
>
> See the example below, based on your self contained example.
>
> Meanwhile, I will think about a more elegant and permanent solution.
>
> Jacob.
>
> ###################
>
> require(raster)
> require(gdistance)
>
> #Create sample rasters
> roads <- raster(nrows=10, ncols=10)
> altitude <- raster(nrows=10, ncols=10)
> roads <- setValues(roads, round(runif(10*10)) )  ; plot(roads)
> altitude <- setValues(altitude, runif(10*10))        ; plot(altitude)
>
> #Define transition layers
> tr.roads <- transition(roads, transitionFunction=function(x) x[1]*x[2]*60,
> 8)  #  driving at 60km/h is possible iff both cells are roads (is this
> correct)?
> #Yes, this is a sensible approach.
> tr.alt <- transition(altitude, transitionFunction=function(x)
> abs(x[1]-x[2]), 8, symm=F)  #just a sample function.
> #Take a look a the vignette for a more realistic (but not state-of-the-art)
> approach.
>
> #Now how to combine them by choosing the "fastest" transportation method?
> tr.stacked <- max(tr.roads, tr.alt)  #does not work
>
> #Alternative approach using indexing
> adjacencyFromTransition <- function(transition) #Probably a function I need
> to include in the package
> {
>     transitionMatr <- as(transition,"sparseMatrix")
>     transition.dgT <- as(transitionMatr,"dgTMatrix")
>     adjacency <- cbind(transition.dgT at i+1,transition.dgT at j+1)
>     return(adjacency)
> }
>
> adj <- adjacencyFromTransition(tr.roads)
>
> #Now use the index together with pmax to do what you want
> tr.alt[adj] <- pmax(tr.roads[adj], tr.alt[adj])
>
> ###############
>
> ________________________________
> From: Thomas Chadefaux <chadefaux at gmail.com>
> To: r-sig-geo at r-project.org
> Sent: Monday, 26 September 2011, 17:37
> Subject: [R-sig-Geo] gdistance -- How to combine transition Layers?
>
> Dear all,
>
> This is my first post here, so I thank you in advance for your help.
>
> My goal is to calculate the cost of traveling (friction) between two
> geographical points, given underlying "friction" rasters.
>
> Suppose for example that my data is composed of two rasters, one
> binary for roads (a given cell has a road or not), and another,
> continuous, for, say, altitude. Using the "gdistance" package, one can
> calculate a transition function between cells. The resulting
> transitionLayers can be combined by addition or multiplication (see
> http://cran.r-project.org/web/packages/gdistance/vignettes/gdistance-vignette.pdf).
>
> However, what I would need is to combine them using some kind of max
> function. In other words, the transition to use would be roads if
> there is one, otherwise by foot (in which case the altitude transition
> layer should be used). Here is a working example that shows where I
> get stuck:
>
>
> require(raster)
> require(gdistance)
>
> #Create sample rasters
> roads <- raster(nrows=10, ncols=10)
> altitude <- raster(nrows=10, ncols=10)
> roads <- setValues(roads, round(runif(10*10)) )  ; plot(roads)
> altitude <- setValues(altitude, runif(10*10))        ; plot(altitude)
>
> #Define transition layers
> tr.roads <- transition(roads, transitionFunction=function(x)
> x[1]*x[2]*60, 8)  #  driving at 60km/h is possible iff both cells are
> roads (is this correct)?
> tr.alt <- transition(altitude, transitionFunction=function(x)
> x[1]-x[2], 8, symm=F)  #just a sample function.
>
> #Now how to combine them by choosing the "fastest" transportation method?
> tr.stacked <- max(tr.roads, tr.alt)  #does not work
>
> Thank you very much in advance.
> Thomas
>
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