Intersects SpatialLine and SpatialPolygon objects and produces total line length and density per polygon ID.

spLine.ld.R

# Calculate total line length and density for a given set of polygons
#
# Intersects SpatialLine and SpatialPolygon objects, calculates individual line length,
# appends to dataframe extracted from intersect object and summarises by polygon ID.
# If density is not required, do not specify area parameter `a`.
#
# The function assumes a spatial projection system where distance is given in metres.
#
# E.g.
# sp1 = object of class SpatialLine/SpatialLineDataFrame
# sp2 = object of class SpatialPolygon/SpatialPolygonDataFrame
# id = name of  polygon column for summary. Currently requires a-priori insertion into the function
# a = area of polygon (default = 0)
#
# E.g.
# df <- len.dens(lines, polygon, id = t$grouping, a = pi*4.58^2)
require(rgeos)
require(plyr)

spLine.ld <- function(sp1, sp2, id, a = 0){
  r <- raster::intersect(sp1, sp2)
  y <- gLength(r, byid = TRUE)
  t <- r@data
  t["r_length"] <- y/1000
  if(a == 0){
  dat <- ddply(t,.(FID_1),summarize,r_length=sum(r_length))
  } else{
    dat <- ddply(t,.(FID_1),summarize,r_length=sum(r_length)) 
    dat["r_dens"] <- dat$r_length / a
  }
  dat
}