[R-sig-Geo] Error in function to negative buffer a SpatialPolygonsDataFrame - 'names' attribute must be the same length as the vector

[Jon Olav Skoien]

Emma,

I don't know what has changed in rgeos, anyway, your current problem 
seems to be the way you assign the results of gBuffer to a list. gBuffer 
will frequently return NULL (probably the vanishing polygons), which 
removes this element from the list if you try to assign. A simplified 
example:

aa <- vector("list",10)
for (i in 1:10) {
   aa[[i]] <- NULL
   print(paste(i, length(aa)))
}
aa

You can for example try to replace the assignment with
   bb <- gBuffer(spdf[i,], width=widths[i], capStyle="ROUND")
     if (!is.null(bb)) buffs[[i]] = bb
  although you will also have to update your Filter function, as bbox 
will not work on NULL-objects.

Best wishes,
Jon



On 04-Apr-14 16:20, Brooks E.G.E. wrote:
> Dear List Members
>
> Several months ago I wrote a code to buffer a SpatialPolygonsDataFrame. Each attribute of the SPDF needed to be buffered by a different amount (defined by one of the columns within the SPDF), some of which were negative. Some of the negative buffers were large enough to warrant the vanishing of that particular polygon. Since gBuffer on its own could not deal with these aspects, the code included a function which would buffer each polygon individually, add the outputs to a list, remove any features which no longer existed, then convert it back to a SPDF. However I have come back to my code and it no longer works with negative buffers, even on the original datasets. It now produces the following error:
>
> Error in names(buffs)[i] <- ID[i] :
>    'names' attribute [1598] must be the same length as the vector [1597]
>
> even though the length of the SPDF is 1747. It does however still work for positive only buffers. The only thing I can think of is that gBuffer has been updated and now deals with vanishing features differently but I have not been able to adapt the code to make it work. Since all elements of the function come from the same object I don't understand how attributes are ending up at different lengths. Also, if I manually do the function step by step, the attributes remain at 1747 and I am not able to reproduce the same error so I am very confused.
>
> Here is a reproduction of the code:
>
> library(Guerry)
> data(gfrance)
>
> bufferby <- function(spdf,widths,ID){
>    library(rgeos)
>    #Create empty buffer list based on spdf
>    buffs<-vector("list",length(spdf))
>    #For every item in spdf,
>    for(i in seq_len(length(spdf))){
>      #Loop buffer each polygon individually based on widths, and add to 'list'
>      buffs[[i]] <- gBuffer(spdf[i,], width=widths[i], capStyle="ROUND")
>      #Name each object in list by ID
>      names(buffs)[i] <- ID[i]
>    }
>    #Filter results by those that worked (i.e. exclude any 'vanished' buffers)
>    buffs=Filter(function(x){!is.null(bbox(x))},buffs)
>    #Convert the filtered results to SP
>    buffsSP<-SpatialPolygons(lapply(1:length(buffs), function(i) {
>      Pol <- slot(buffs[[i]], "polygons")[[1]]
>      #Add names to be able to link to other datasets
>      slot(Pol, "ID") <- names(buffs)[i]
>      Pol
>    }))
>    #Project output (SP) based on input (spdf)
>    proj4string(buffsSP)<-CRS(projection(spdf))
>    buffsSP
> }
>
> #Create widths defining column, including negatives
> gfrance$buffwid<- runif(length(gfrance), min=-200000, max=20000)
>
> newbuffs<-bufferby(gfrance,gfrance$buffwid,gfrance$Department)
> plot(newbuffs)
>
> If I define "widths" as a column with only positive values it works fine, but it no longer works if any of the negative features vanish.
>
> N.B. This function should (and did) also work for SpatialPoints.
>
> Many thanks for any help, and your time.
>
> Emma
>
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>
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-- 
Jon Olav Skøien
Joint Research Centre - European Commission
Institute for Environment and Sustainability (IES)
Land Resource Management Unit

Via Fermi 2749, TP 440,  I-21027 Ispra (VA), ITALY

jon.skoien at jrc.ec.europa.eu
Tel:  +39 0332 789205

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