Calculate line density within a buffer from a raster

Question

Is there a way to calculate road density (km/km²) within a buffer around spatial lines? The roads are represented by pixels (1 pixel = 625 m²) in a raster. So I began to convert the road pixels into polylines by using the function rasterToContour (package raster). Then, I'm thinking of calculating total length of lines within the buffer (in km) and the buffer area (in km²).

r <- raster(rast_path)
x <- rasterToContour(r)

Here is a reproducible example:

## To create raster:
library(raster)
library(rgeos)
r <- raster(ncols=90, nrows=50)
values(r) <- sample(1:10, ncell(r), replace=TRUE)

## Road raster
r[r[] < 10] <- 0
r[r[] >= 10] <- 1
plot(r)

## To create spatial lines 
line1 <- rbind(c(-125,0), c(0,60))
line2 <- rbind(c(0,60), c(40,5))
line3 <- rbind(c(40,5), c(15,-45))
line1_sp <- spLines(line1)
line2_sp <- spLines(line2)
line3_sp <- spLines(line3)

## To create buffer around lines
line2_buff <- gBuffer(line2_sp, width=20)
plot(line2_sp,add=T)
plot(line2_buff,add=T)

Answer 1

You're looking for road length (kilometers) divided by buffer area (square kilometers), right? To calculate road length, you can use your method with rasterToContour(), although that isn't reproducible with the example you provided.

But to calculate area of the buffer in square kilometers, you can do: n <- length(extract(r, line2_buff)) to get the n number of pixels in the buffer, which are 625 m^2 each. The conversion factor you need is 1 km^2 = 1,000,000 m^2. All together, area of the buffer in km^2 is given by:

length(extract(r, line2_buff)) * 625 / 1000000