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I am working on a hydrological model and I needed a DEM for the entire watershed, so I had to merge multiple DEM tiles in order to cover all of the watershed. There were 34 tiles to merge in total. I did the merge by using a virtual raster (GDAL - build virtual layer) and then saving it.

The problem is that any processing takes a very long time to finish, it can take up to a few days to complete and the merged DEM file alone is around 100gb.

Should I keep each tile separated or is there a better way to work with a single large DEM file?

Francis Lapointe
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    Hi there. Some extra information can help. The data is of type Float32 or Float64 or other? In what format are they saved? Do they have some kind of compression? – Gabriel De Luca Sep 29 '21 at 15:26
  • Hi, thanks for the reply! The data type is Float32 and the format is GeoTiff. When building the virtual raster, I selected the "Lowest" option in the "Resolution" menu. Other than that there was no other compression applied. Let me know if there are other helpful information missing! – Francis Lapointe Sep 29 '21 at 15:33
  • With "any processing" do you mean processes that deal with the whole watershed, or also processes that handle only a small area? Did you create your big DEM tiff as tiled -co tiled=yes? – user30184 Sep 29 '21 at 16:18
  • Just clipping the DEM file was taking a lot of time, otherwise there is also the slope tool and the fill sinks. These tools were used on the entire watershed (the big DEM file). I didn't use the "-co tiled=yes", do I add this in the "additional command-line parameters"? – Francis Lapointe Sep 29 '21 at 17:23
  • That's the place to add the option. Why tiled is better than striped when image gets wider see the comment in https://gis.stackexchange.com/questions/27160/geotiff-format-tile-and-overview-confusion. – user30184 Sep 29 '21 at 22:40
  • What's the size of the watershed? and what is the resolution of the DEM tiles? As a first guess: reduce the target resolution (when you run gdalbuildvrt). – Micha Sep 30 '21 at 04:53

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