Creating a buffer for a line raster using a polygon boundary

Question

I have a raster(plot 1) for which I have to create a buffer zone using a polygon (Plot 2). The raster cell size is 10m and the polygon width (d), as indicated is constant and is 15m, so the buffered raster will need to include either 1 more adjacent cell (one side) or 2 adjacent cells (both sides) and should assign the same value (X) as the original cell for the new cell(s) as indicated. How can I do this in R? I thought I can do this by using raster::buffer and raster::extend, but I realised it's not a straightforward task.

I have to do this for many cases and sometimes the width of the polygon is not constant, so a more general solution would be great.

Answer 1

Your question is not very clear and you do not provide example data. My understanding is that you want to assign values to raster cells that are covered by a polygon, but only values that are currently NA, and that they should be similar to the nearby raster cell values. Your image suggests that you rasterized a line and the whole thing might be easier if you backtracked a bit --- but you provide insufficient information to comment. If my understanding of your goal is right, you could do something like this:

Example data

library(raster)
r <- raster(resolution=5)
values(r) <- 1:ncell(r)
cds <- rbind(c(-160,-20), c(-140,55), c(10, 0), c(140,60))
line <- spLines(cds)
r <- mask(r, line)
poly <- buffer(line, 10)

Solution: assign similar values to neighboring cells. The size of the window depends on the polygon used, but 5x5 works for the example. Then use mask to remove cells that are outside the polygon

rr <- focal(r, matrix(1, 5, 5), fun=mean, na.only=TRUE, na.rm=TRUE)
x <- mask(rr, poly)

Instead of with raster you can also do this with terra (it is faster)

library(terra)
r <- rast(resolution=5)
values(r) <- 1:ncell(r)
cds <- rbind(c(-160,-20), c(-140,55), c(10, 0), c(140,60))
line <- vect(cds, "lines", crs=crs(r))
r <- mask(r, line)
poly <- buffer(line, 1000000)

rr <- focal(r, matrix(1, 5, 5), fun=mean, na.only=TRUE, na.rm=TRUE)
x <- mask(rr, poly)