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I am trying to generate a large set of binary rasters from a multi polygon shapefile. My snap raster has a large pixel, 0.5 x 0.5 degrees. I don't have major problems rasterizing de large polygons, but, for the small ones, I am writing empty raster (all 0). I am wondering if there is a tolerance parameter in rasterize function by which I will be able to assign 1 to every pixel touched by a polygon (even if the % of the polygon touched is very small).
This is the part of the code for (i in 1:length(shape)) { shape.r<-rasterize(shape[i,],snap, background=0) writeRaster(shape.r, filename = paste(shape[i,]$binomial, sep=""), format = "GTiff", overwrite = T) }

Thanks! Javier.

Javi Nori
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1 Answers1

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Here is some example data

f <- system.file("ex/lux.shp", package="terra")
v <- vect(f)
x <- lapply(1:nrow(v), \(i)rescale(v[i,], 0.2))
vv <- vect(x)
r <- rast(v, ncols=10, nrows=10)
b <- as.lines(r)

The standard rasterize method misses a lot of the polygons if they are small relative to the cell size.

x <- rasterize(vv, r, "ID_2")
plot(x)
lines(b, col="light gray")
lines(vv)

enter image description here

The argument touches=TRUE can help in this case

xx <- rasterize(vv, r, "ID_2", touches=TRUE)
plot(xx)
lines(b, col="light gray")
lines(vv)

enter image description here

But if they polygons are even smaller, they could still be missed. One approach to deal with that is to also rasterize the centroids of the polygons (rasterize probably should have an argument to do this automatically).

cnt <- centroids(vv)
# leaving out touches=T otherwise the example is not interesting
# with these data
x <- rasterize(vv, r, "ID_2")
y <- rasterize(cnt, r, "ID_2")
z <- cover(x, y)

plot(z)
lines(b, col="light gray")
lines(vv)

enter image description here

You can also consider using

x <- rasterize(vv, r, cover=TRUE)
x <- x > 0
Robert Hijmans
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  • Great! Thank you very much, Robert! – Javi Nori Nov 12 '21 at 19:04
  • Hi Again! I tried with terra::rasterize (small=T , touches=T ), but I still have the same problems, for very small polygons I continue writing rasters full of 0... any other solution?? Thank you very much in advance. JAvier. – Javi Nori Nov 16 '21 at 15:35
  • I have expanded my answer – Robert Hijmans Nov 16 '21 at 16:56
  • Great Robert! Thank you. I believe that in this case, I would have problems with the large polygons, but, perhaps I could try, firstly converting the polygon to points using my snap raster, I this works, I can then rasterize the points file! do you think it could work? – Javi Nori Nov 16 '21 at 17:36
  • What would the problems with the large polygons" be? – Robert Hijmans Nov 16 '21 at 17:43
  • Hi Robert! Finally, I believe I was able to solve the problem... I used firstly the function rasterize, then I used a function to identify all the rasters full of 0, and finally, I rasterized these very small polygons using the centroids. Thank you very much for your time. – Javi Nori Nov 17 '21 at 12:57
  • That is what I do in my answer by employing `cover` (I use NA, but you can use zero as well) – Robert Hijmans Nov 17 '21 at 19:38