Batch processing DEM rasters in QGIS/GDAL

Question

I am attempting to add relief shading to map using .asc raster files, following this tutorial. It works well, but I have only tried it on single tile of raster data which is 20x20km. I want to generate the shading for a much larger area, which means multiple tiles.

I understand I can input GDAL commands in the Hillshade/DEM dialogue in QGIS, and the tutorial suggests this means I can perform a batch operation, but I don't know enough (read any) GDAL to do this.

I am using QGIS 1.9 on Ubuntu (though I do have a Windows installation as well).

Any ideas?

EDIT: I have written the following script, demraster.sh:

!/bin/sh 
for f in *.asc 
do
echo "Processing %f" 
gdaldem hillshade %f -z 1.0 -s 1.0 -az 315.0 -alt 45.0 
done

I have set it to be executable with chmod, and run it with sh demraster.sh but get a syntax error:

Syntax error: word unexpected (expecting "do")

Answer 1

Looking at the dialog here, you might have some odd POSIX user permission stuff getting in the way. Or possibly DOS line endings (rather than UNIX). Rather than making a file, just do the loop from the command prompt, by typing it out in the terminal. I do this routinely, without the need of making a shell script file.

Typing this the first time, you should see the prompt character in the first column change from $ to > (don't type these!) until the end of the loop block:

$ for f in *.asc
> do
> echo "Processing $f"
> gdaldem hillshade  -z 2.0 -az 345.0 -alt 50.0 $f $f-HS.tif
> done

(Note: this is identical to yours, except I replace %f with $f.) After this, you can type the up key to see the previous command.

for f in *.asc; do echo "Processing $f"; gdaldem hillshade  -z 2.0 -az 345.0 -alt 50.0 $f $f-HS.tif; done

It is shown as a one-line condensed version with semi-colons, which you can use and modify (if needed) for further work from the command prompt.

Answer 2

This should do the trick. Make a text file with the following lines:

for f in *.asc
do
  echo "Processing $f"
gdaldem hillshade  -z 2.0 -az 345.0 -alt 50.0 $f $f-HS.tif
done

Save it with the extension .sh (for example "hillshade.sh"). Drop the file in the directory with your ASC rasters and then run it from a terminal: "sh hillshade.sh".

N.

Answer 3

You probably want the use the shell and gdaldem:

http://gdal.org/gdaldem.html

gdaldem hillshade in.asc out.tif

Answer 4

My guess would be that you are having issues with line endings - given that we have tried almost everything else.

I would use the script provided by @BradHards (copied below) and manually retype it on your system in either vi or nano. My suggestion would be nano as you do not need to learn vi to get this working.

Step by step would be to:

1. Open a terminal and cd to the directory with your *.asc files.
2. execute the command nano
3. Retype the code below - do not cut and paste.
4. control-x to exit, name the file batch_dem.sh or something, and save it.
5. chmod +x batch_dem.sh

6. ./batch_dem.sh

   #!/bin/sh 
   for f in *.asc;
   do
   echo "Processing %f";
   gdaldem hillshade %f -z 1.0 -s 1.0 -az 315.0 -alt 45.0;
   done
   

Answer 5

There are 2 ways (i know) to handle this:

1. Using Bash commands (for loop)[only in linux]

2. Using scripting languages like Python (for loop)[every system that installed Python]

Here is the code. You can modify it easily for linux

import os

pathOfDems = 'F:/Gdem'

pathForHill = 'F:/Hillshade'

pathToGdalDem = 'C:/OSGeo4W/bin/gdaldem.exe'

dliste = os.listdir(pathOfDems)

for i in range (0,len(dliste),1):

os.system(pathToGdalDem + ' hillshade ' + pathOfDems + os.sep + dliste[i] + '    ' +  pathForHill + os.sep + 'Hillshade_' + dliste[i][8:-8] + '.tif')