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I'm generating contours from four 5x5 degree SRTM DEMs in southeastern Australia. I'm using gdalwarp to reproject the DEM, then gdal_contour to generate contours.

First, I generated contours for each block separately. The shapefiles were 9, 45, 73 and 270 MB. Then I merged the source TIFFs, and generated a single contour file. It's 2.0GB (that is, more than 4x the size of the combined individual contours).

Why is this? Here are the two scripts:

Individual contours:

for f in srtm_66_19 srtm_65_19 srtm_66_20 srtm_65_20
do
gdalwarp -s_srs EPSG:4326 -t_srs EPSG:3785 -r bilinear $f.tif $f-3785.tif 

# contours
gdal_contour -a elev -i 20 $f-3785.tif $f-3785-contour.shp

done

Merged contours:

gdal_merge.py srtm_66_19.tif srtm_65_19.tif srtm_66_20.tif srtm_65_20.tif -o srtm.tif
f=srtm
gdalwarp -s_srs EPSG:4326 -t_srs EPSG:3785 -r bilinear $f.tif $f-3785.tif 
gdal_contour -a elev -i 20 $f-3785.tif $f-3785-contour.shp

And the complete file listing:

-rw-rw-r-- 1 ubuntu ubuntu  20M Sep  9 17:36 new/srtm-3785-contour.dbf
-rw-rw-r-- 1 ubuntu ubuntu  399 Sep  9 17:08 new/srtm-3785-contour.prj
-rw-rw-r-- 1 ubuntu ubuntu 2.0G Sep  9 17:36 new/srtm-3785-contour.shp
-rw-rw-r-- 1 ubuntu ubuntu 7.6M Sep  9 17:36 new/srtm-3785-contour.shx
-rw-rw-r-- 1 ubuntu ubuntu 404M Sep  9 17:07 new/srtm-3785.tif
-rw-rw-r-- 1 ubuntu ubuntu 8.9M Sep  9 13:34 srtm_65_19-3785-contour.shp
-rw-rw-r-- 1 ubuntu ubuntu  68M Sep  9 12:28 srtm_65_19-3785.tif
-rw-rw-r-- 1 ubuntu ubuntu  69M Sep  9 12:28 srtm_65_19.tif
-rw-rw-r-- 1 ubuntu ubuntu  73M Sep  9 13:38 srtm_65_20-3785-contour.shp
-rw-rw-r-- 1 ubuntu ubuntu  67M Sep  9 12:28 srtm_65_20-3785.tif
-rw-rw-r-- 1 ubuntu ubuntu  69M Sep  9 12:28 srtm_65_20.tif
-rw-rw-r-- 1 ubuntu ubuntu  45M Sep  9 12:28 srtm_66_19-3785-contour.shp
-rw-rw-r-- 1 ubuntu ubuntu  68M Sep  9 12:28 srtm_66_19-3785.tif
-rw-rw-r-- 1 ubuntu ubuntu  69M Sep  9 12:28 srtm_66_19.tif
-rw-rw-r-- 1 ubuntu ubuntu 270M Sep  9 13:37 srtm_66_20-3785-contour.shp
-rw-rw-r-- 1 ubuntu ubuntu  67M Sep  9 12:28 srtm_66_20-3785.tif
-rw-rw-r-- 1 ubuntu ubuntu  69M Sep  9 12:28 srtm_66_20.tif

Why so big? Is there anything I can do about this? I'd prefer to work with merged files as they're easier to manage in TileMill.

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I would like to put this as a comment, but I don't have enough reputation (???).

Anyways, you could keep using the individual DEM files if you created a .vrt file with gdalbuildvirt.

On the other hand, the problem I got when creating contours from individual files was that they look funny where DEMs meet. I tried to create them from the .vrt file, but my are was too big and run out of memory.

  • Oh, what do you mean? You can have a VRT file that kind of spans several different source Shapefiles, so you don't need to actually merge them? – Steve Bennett Mar 15 '17 at 5:58
  • No, you can have VRT that spans several raster and then apply contours on that. – Marcos Dione Mar 15 '17 at 12:31

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