Xtykutl

I have a polygon object of a transect line that I would like to divide into 3 equal parts that are one-after-the-other along the transect line. So far I have only been able to slice the polygon vertically, which results in non-continuous parts.

Here is a reproducible example using Harvard Forest data, available through the following link

After you have downloaded and unzipped the data, you will see a folder called HARV. In this folder is the shapefile, HARV_roads.shp

Import HARV_roads.shp

lines_HARV <- readOGR("/Your file path/NEON-DS-Site-Layout-Files/HARV/HARV_roads.shp")

Get footpath lines only

footpath_HARV <- lines_HARV[lines_HARV$TYPE == "footpath",]

Plot the footpath

plot(footpath_HARV,

     lwd=6,

     main="NEON Harvard Forest Field Siten Footpath")

Create a thicker polygon object from the footpath line using buffer, to simulate a transect line with X width

require(raster)

footpath_buffer <- buffer(footpath_HARV,width=40)

Plot footpath_buffer

plot(footpath_buffer)

Chop the buffer into 3 equal parts using Barry Rowlingson's code (below) from this post:

makeVchopper <- function(pol){

  bb = bbox(pol)

  delta = (bb[2,2] - bb[2,1])/10

  xmin = bb[1,1]-delta

  ymin = bb[2,1]-delta

  ymax = bb[2,2]+delta



  choppoly = function(xmax){

    readWKT(sprintf("POLYGON((%s %s, %s %s, %s %s, %s %s, %s %s))",

                    xmin,ymin, xmin,ymax, xmax,ymax, xmax,ymin,

                    xmin,ymin))

  }

  choppoly

}



slicer <- function(pol, xmin, xmax){

  bb = bbox(pol)

  delta = (bb[2,2] - bb[2,1])/10

  ymax = bb[2,2] + delta

  ymin = bb[2,1] - delta

  r = readWKT(sprintf("POLYGON((%s %s, %s %s, %s %s, %s %s, %s %s))",

                      xmin,ymin, xmin,ymax, xmax,ymax, xmax,ymin, xmin,ymin))

  gIntersection(pol,r)

}



chop_thirds <- function(pol, fractions=c(1/3, 2/3)){

  chopper = makeVchopper(pol)

  bb = bbox(pol)

  xmin = bb[1,1]

  xmax = bb[1,2]



  totalArea = gArea(pol)



  chopped_area = function(x){

    gArea(gIntersection(chopper(x),pol))

  }



  edges = lapply(fractions, function(fraction){

    target = totalArea * fraction

    target_function = function(x){

      chopped_area(x) - target

    }

    uniroot(target_function, lower=xmin, upper=xmax)$root

  })



  xdelta = (xmax-xmin)/10

  chops = matrix(c(xmin-xdelta, rep(edges,rep(2,length(edges))),

                   xmax+xdelta), ncol=2, byrow=TRUE)

  apply(chops, 1, function(edges){

    slicer(pol, edges[1], edges[2])

  })



}

Divide footpath_buffer into 3 equal parts:

parts.footpath <- chop_thirds(footpath_buffer)

Plot footpath_buffer and the 3 equal parts

plot(footpath_buffer)

plot(parts.footpath[[1]], add=TRUE, col=1,border=F)

plot(parts.footpath[[2]], add=TRUE, col=2,border=F)

plot(parts.footpath[[3]], add=TRUE, col=3,border=F)

footpath_buffer is now divided into 3 equal parts, but the parts are broken up. I am hoping to achieve something like this, where each part is continuous along the transect: