Buffer spatialline on one side [closed]

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I have a SpatialLinesDataFrame of many transects that I would like to buffer on only the North side of line using R. Does anyone know of tool that mimics the 'Side Type' option for the Buffer tool in arcgis? Thanks!

Answer 1

If your transects are line segments that are monotonically increasing in X (and if not, I don't know how you can define what is north and what is south of a line) you can use this code. Basically it constructs the buffer with gBuffer and intersects that with a polygon created by extending the line segment south.

northbuff <- function(l1, width){
    if(length(l1)!=1){
        stop("line is not a single line element")
    }

    lines = l1@lines
    if(length(l1@lines)!=1){
        stop("line element is not a single line string")
    }

    bb = bbox(l1)
    xy = coordinates(l1@lines[[1]])[[1]]

    if(any(diff(xy[,1])<=0)){
        stop("x coord not monotone increasing")
    }

    xy = xy[order(xy[,1]),]
    nx = nrow(xy)

    xm = bb[1,1]-width*2

    ym = bb[2,1]-width*2

    xp = bb[1,2] + width*2

    coords = rbind(xy,
        c(xp,xy[nx,2]),
        c(xp,ym),
        c(xm,ym),
        c(xm,xy[1,2]),
        c(xy[1,1],xy[1,2])
        )
    p = SpatialPolygons(list(Polygons(list(Polygon(coords)),ID=1)))

    b = gBuffer(l1, capStyle="FLAT", width=width)

    gDifference(b,p)

}

Test:

> require(sp)
> require(rgeos)
> l1 = readWKT("LINESTRING(0 0,1 5,4 5,5 2,8 2,9 4)")
> plot(northbuff(l1,.2))
> plot(l1,add=TRUE,col="blue",lwd=2)

You'll have to loop over your data frame to do this for each SpatialLines component.

If your transects are single line segments (ie a straight line from x,y to x',y') then its trivial and doable without rgeos. If your transects aren't strictly increasing in x-coordinate then you'll have to figure out how to define north and south of a line. This code could be easily modified to produce two polygons as output, which you could then input into a function to decide which was more 'North'.