Check for intersection of raster and vector in R

Question

I have a rasterstack and a vector-geometry (sf-object). The raster's values are either ǸA or some integer. I would like to check if the raster (in the best case just as a binary response) has any intersection with the shape in the points that are non-NA in the raster. And I don't want to use the bounding box of the raster. I thought about polygonzing the raster where the values are not NA. But this seems a lot of effort and space. I'm not asking for any specific code, but maybe an approach that could be used in R

Answer 1

You can use extract from the raster package to get a list of overlapping raster and polygon values, and then can check the extracted values - if there are any NAs then the sum of the values will be NA:

Reproducible example, with a raster half integer 1s and half NAs:

library(sf)
library(raster)

r <-
  raster(
    nrows = 50, ncols = 50,
    xmn = 0, xmx = 5, ymn = 0, ymx = 5,
    crs = CRS("+init=epsg:4326"),
    vals = c(rep(1, 1250), rep(NA, 1250))
  )


# Polygon without overlap of NA area:
p1 <-
  data.frame(id = 1, wkt = 'POLYGON((2 3, 2.5 3, 2.5 3.5, 2 3.5, 2 3))')
p1 <- st_as_sf(p1, wkt = 'wkt', crs = 4326)

sum(unlist(extract(r, p1))) # Result [1] 25

# Polygon totally overlapping with NA area:
p2 <-
  data.frame(id = 1, wkt = 'POLYGON((0 0, 0 0.5, 0.5 0.5, 0.5 0, 0 0))')
p2 <- st_as_sf(p2, wkt = 'wkt', crs = 4326)

sum(unlist(extract(r, p2))) # Result [1] NA

Answer 2

I decided to go the following way. There is a package called velox which does extraction of raster values based on polygons really fast. For some reason it worked better for me using sp than sf-objects. So here's more or less what I did:

rast.norm = raster(rasts[i]) # load it first as RasterLayer
rast.trim = trim(rast.norm) # trim that raster to delete outer NAs
shape.crp = st_crop(shape, rast.trim) 
shape.sp = sf::as_Spatial(shape.crp)
rast.vx = velox(rast.trim) # load runout raster as velox raster
ex.vx = rast.vx$extract(shape.sp) 
matr = do.call(rbind, ex.vx) # rowbind all matrices 
print(sum(matr[,1][!is.na(matr[,1])]))

If the sum is 0 or the crop-step doesn't work it means there is no intersection. I think maybe isn't the fastest way, but for me it worked.