Merging multiple rasters in R

Question

I've been trying to find a time-efficient way to merge multiple raster images in R. These are adjacent ASTER scenes from the southern Kilimanjaro region, and my target is to put them together to obtain one large image.

This is what I got so far (object 'ast14dmo' representing a list of RasterLayer objects):

# Loop through single ASTER scenes for (i in seq(ast14dmo.sd)) {   if (i == 1) {     # Merge current with subsequent scene     ast14dmo.sd.mrg <- merge(ast14dmo.sd[[i]], ast14dmo.sd[[i+1]], tolerance = 1)   } else if (i > 1 && i < length(ast14dmo.sd)) {     tmp.mrg <- merge(ast14dmo.sd[[i]], ast14dmo.sd[[i+1]], tolerance = 1)     ast14dmo.sd.mrg <- merge(ast14dmo.sd.mrg, tmp.mrg, tolerance = 1)   } else {     # Save merged image     writeRaster(ast14dmo.sd.mrg, paste(path.mrg, "/AST14DMO_sd_", z, "m_mrg", sep = ""), format = "GTiff", overwrite = TRUE)   } } 

As you surely guess, the code works. However, merging takes quite long considering that each single raster object is some 70 mb large. I also tried Reduce and do.call, but that failed since I couldn't pass the argument 'tolerance' which circumvents the different origins of the raster files.

Anybody got an idea of how to speed things up?

Answer 1

You can use do.call

ast14dmo.sd$tolerance <- 1 ast14dmo.sd$filename <- paste(path.mrg, "/AST14DMO_sd_", z, "m_mrg.tif", sep = "") ast14dmo.sd$overwrite <- TRUE mm <- do.call(merge, ast14dmo.sd) 

Here with some data, from the example in raster::merge

r1 <- raster(xmx=-150, ymn=60, ncols=30, nrows=30) r1[] <- 1:ncell(r1) r2 <- raster(xmn=-100, xmx=-50, ymx=50, ymn=30) res(r2) <- c(xres(r1), yres(r1)) r2[] <- 1:ncell(r2)  x <- list(r1, r2) names(x) <- c("x", "y") x$filename <- 'test.tif' x$overwrite <- TRUE m <- do.call(merge, x) 

Answer 2

The 'merge' function from the Raster package is a little slow. For large projects a faster option is to work with gdal commands in R.

library(gdalUtils) library(rgdal) 

Build list of all raster files you want to join (in your current working directory).

all_my_rasts <- c('r1.tif', 'r2.tif', 'r3.tif') 

Make a template raster file to build onto. Think of this a big blank canvas to add tiles to.

e <- extent(-131, -124, 49, 53) template <- raster(e) projection(template) <- '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs' writeRaster(template, file="MyBigNastyRasty.tif", format="GTiff") 

Merge all raster tiles into one big raster.

mosaic_rasters(gdalfile=all_my_rasts,dst_dataset="MyBigNastyRasty.tif",of="GTiff") gdalinfo("MyBigNastyRasty.tif") 

This should work pretty well for speed (faster than merge in the raster package), but if you have thousands of tiles you might even want to look into building a vrt first.