Clipping multiple rasters by matching polygons from SPDF in R

Question

I have about 50.000 small glacier rasters (rectangular shape) that need to be clipped to the glacier's extent. I have the glacier polygons stored in a large SpatialPolygonsDataFrame.

I want to use parallel computing or else it would take forever, I think. For previous tasks I've successfully used the mcmapplyfunction, but I am open for other approaches.

My (admittedly rudimentary) code so far is:

filenames <- list.files("/.../RGI60-13_reproj/", pattern="*.tif", full.names=F)
filelocations <- list.files("/.../RGI60-13_reproj/", pattern="*.tif", full.names=T)
glaciers <- readOGR("/.../13_rgi60_CentralAsia.shp",verbose=TRUE)

fun_clip <- function(filelocations, filenames, glaciers){
  r <- raster(filelocations)
  r <- crop(r,glaciers) # here I need to clarify the corresponding shp in the SPDF
  writeRaster(r, paste0("/.../RGI60-13_crop/",filenames))
}

mcmapply(fun_proj, filelocations, filenames, mc.cores = 50)

How can I give the crop-function the right iterative arguments? filelocationis of the same length as glaciers, so in a for-loopI would use something like r <- crop(r,glaciers[i]), but how do I pass the iteration in my kind of function? What would be the way to introduce the i, so to speak?

Answer 1

"filelocation is of the same length as glaciers" and in the same order? In which case:

for(i in 1:length(filelocations)){
  r = raster(filelocations[i]) # get the ith file name
  g = glaciers[i,] # get the i'th row of `glaciers`.
  r = crop(r, g) # crop
  writeRaster(r, file.path(outputdest, filenames[i])) # save in outputdest
}