Aslak Grinsted‎ > ‎Misc. Debris‎ > ‎

GDEMv2 geotiff compression

posted Aug 10, 2013, 2:25 PM by Aslak Grinsted   [ updated Aug 10, 2013, 2:51 PM ]
There is no good way to download the entire GDEM dataset. It is just lots of manual work. I assume it is because of datavolume that they put artificial restrictions amd annoying hindrances in place. 

There is 22000 tiles covering 1x1deg. Each tile file is a zip containing two geotiffs. A dem file and a num file. Unzipped then this is about 49Mb (or ~1Tb for all files). Zipped they are about 15Mb (or 330Gb for all).

However the geotiffs appear to be uncompressed tifs. I just downloaded the one covering Copenhagen and tried to use GDAL to compress them with these commands:

> gdal_translate -co "COMPRESS=DEFLATE" -co "predictor=2" -co tiled=yes "ASTGTM2_N55E012_dem.tif" "ASTGTM2_N55E012_dem_small.tif"

> gdal_translate -co "COMPRESS=DEFLATE" -co tiled=yes "ASTGTM2_N55E012_num.tif" "ASTGTM2_N55E012_num_small.tif"

The resulting compressed tifs outperform the zipped files:
  • DEM: Before=24.7Mb  |  CompressedTif=3.14Mb
  • NUM: Before=24.7Mb  |  CompressedTif=1.29Mb
  • Total: Before=49.4Mb | Zipped=5.2Mb  |  CompressedTif=4.4Mb
I also tried with N080W046: ZippedTotal=16.3Mb vs. CompressedTifs=12.3Mb

Anyway I don't understand why they don't distribute compressed geotiffs. It is certainly better than zipping it. 

I've also tried the same with the GIMP Greenland DEM and it also gives very good compression. Probably a lot better actually because of the smooth topography of the ice sheet.  

I used gdal_translate included in FW_TOOLS.