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

[Barry Rowlingson]

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