# [R-sig-Geo] [R-sig-geo] Sum lines when they intersect polygons

Michael Sumner mdsumner at gmail.com
Sat Apr 9 01:25:08 CEST 2011

```If you really just need a grid then pixellate.psp in spatstat can do
this, pass in the "length" with the weights argument.

On Sat, Apr 9, 2011 at 12:32 AM, Barry Rowlingson
<b.rowlingson at lancaster.ac.uk> wrote:
> Slow and dirty method:
>
> lineLengthIntersects <- function(gridpolys,linegeom){
>  ngrids = length(gridpolys)
>  ll = rep(0,ngrids)
>  for(i in 1:ngrids){
>    geomint = gIntersection(linegeom,gridpolys[i])
>    if(!is.null(geomint)){
>      ll[i] = SpatialLinesLengths(geomint,TRUE)
>    }
>  }
>  return(ll)
> }
>
>  this returns the line length in km (TRUE in SpatialLinesLengths means
> work in lat-long on a sphere and convert to km) for overlaying
> polygons in gridpolys (a SpatialPolygons object) on a line geometry in
> linegeom (a SpatialLines object).
>
>  there might be a better way of doing it without the loop...
>
> here's my polygrid function for creating SpatialPolygons of a regular
> grid based on x and y - use something like:
>
> grids = polygrid(seq(0,1,len=10),seq(0,1,len=10)) to get a grid
> somewhere near the greenwich meridian on the equator...
>
> polygrid <- function(x,y){
>  nx = length(x)
>  ny = length(y)
>  spp = list()
>  ip = 1
>  for(ix in 1:(nx-1)){
>    for(iy in 1:(ny-1)){
>      xc=x[c(ix,ix+1,ix+1,ix,ix)]
>      yc=y[c(iy,iy,iy+1,iy+1,iy)]
>      spp[[ip]] = Polygons(list(Polygon(cbind(xc,yc))),ID=ip)
>      ip = ip + 1
>    }
>  }
>  SpatialPolygons(spp)
> }
>
>
> Barry
>
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>

--
Michael Sumner
Institute for Marine and Antarctic Studies, University of Tasmania
Hobart, Australia
e-mail: mdsumner at gmail.com

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