I tell you. It always amazes me how much cool stuff you can do with great open source GIS software these days. GDAL is one of those great open source projects that I have just found a great use for (apart from just opening every raster type under the sun in QGIS).

GDAL has the ability to generate contours from a DEM, something that I have always wanted to try for my town but have never been able due to lack of a good DEMs.

Recently we purchased a set of DEMs that cover a large area as part of a study. Each DEM uses a grid size of 1mx1m. Prefect for generating contours.

Using the GdalTools QGIS plugin.

First make sure that you have the latest version of the GdalTools plugin installed (GdalTools should be installed by default with QGIS. If it’s not, search “Gdal” in the plugin installer). Enable the plugin once it’s installed.

Load the DEM into QGIS using the Load Raster icon.

DEM loaded in QGIS

Now head up to the menu Raster->Extraction->Contour

Raster menu in QGIS

Select the settings that you need. For this DEM I am going to generate 250mm contours.

Contour dialog.

Take note of the text area at the bottom of the dialog as that is the exact command sent to GDAL in order to generate the contours. If you take a copy of that you can run it on the command line for batch processing later.

Hit ok.

250mm contours from the DEM

BAM! :)

Using the command line/shell.

Using QGIS for a one off DEM is fine and dandy but what if you have 3000 DEMs that you need to process. To hell with doing that by hand!

Remember the contour tool in QGIS told us the exact command line args to use, so creating a shell script for automating the process is pretty easy.

for f in *.asc
do
  echo "Processing $f"
 gdal_contour -a ELEV -i 0.25 $f $f-250mm.shp
done

The above code will loop though the current directory and process all the DEMs generating 250mm contours for each one. It saves each new contour file as {filename}-250mm.shp. You will need to change *.asc to whatever format your DEM is in.

Copy the above code into a file somewhere and call it generate_contours.sh. This can then be called from the command line using

sh generate_contours.sh

Running sh on Windows

If you’re a windows user you will need to run OsGeo4W Shell in order to use sh.

Loading OsGeo4w shell.

Once the shell is loaded you can just call:

sh generate_contours.sh

Output from running generate_contours.sh

Final remarks

I ran the above sh script on a folder with about 2500 DEMs and it took around 4 hours to complete the whole folder. Of course performance will vary but it seems pretty quick considering.

Once again the possibly to use open source GIS tools to get my work done is amazing.  No expensive software here.

So far I’m not aware of any line smoothing/generalizing abilities using GDAL/OGR.  Although you can import the contours into GRASS and use that: http://grass.osgeo.org/wiki/V.generalize_tutorial

You can also generate the contours using GDAL via Python but that is a topic for another day.

Note:  This post is about getting ecw and mrsid support working in the trunk (qgis-dev) version of QGIS which is installed with the OSGeo4W installer.  Non-dev versions seem to work fine following this method: http://www.qgis.org/wiki/OSGeo4wSetup#Installing_support_for_raster__.2A.ECW_.28ERMapper.29_and_.2A.sid_.28MrSid.29_formats

Getting ECW and MrSID support into QGIS can be a real pain in the butt when you first try; but once you know how it’s easy.

If you try to open a ecw file in QGIS you will get this nice little error message:

What?  I thought QGIS supported ECW files!

Turns out it does, but the team are not able to ship the required libs with the program due to licensing restrictions by ERDAS.   Luckily adding support is not “too” hard.

1. First head over to http://www.erdas.com/products/ERDASECWJPEG2000SDK/Downloads.aspx
2. Click Download Now for the ERDAS ECW/JP2 SDK Desktop Read-Only, Version 4.1 libs,
3. Fill out a bit of registration details and click though all the pages (this is the most painful process I have ever had to go though to get some libs for a program, ERDAS should be ashamed that it is such an effort.)
4. Once you have download ECWJP2SDKSetup_RO_20100920.exe; install it.
5. Copy all the files from C:\Program Files\ERDAS\ERDAS ECW JPEG2000 Read SDK\bin\vc90\win32 into the bin folder of your OSGeo4W install (default is C:\OSGeo4W\bin)
   You can open the OSgeo4W shell and run: copy “C:\Program Files\ERDAS\ERDAS ECW JPEG2000 Read SDK\bin\vc90\win32\*.dll” %OSGEO4W_ROOT%\bin to do the same thing.
6. Launch osgeo4w-setup.exe, the installer that you used to install qgis-dev,  and select  gdal-ecw, gdal17-ecw and gdal-mrsid, gdal17-sid under the libs section. Let it install the needed libs and any dependencies.
7. Open %OSGEO4W_ROOT%\bin and search for qgis-dev.bat; open it with a text editor and add the following line:
   set GDAL_DRIVER_PATH=”%OSGEO4W_ROOT%”\bin\gdalplugins\1.8
   I always insert it just after  SET OSGEO4W_ROOT=
8. Launch qgis-dev.bat
9. Open a ecw

ECW file opened in QGIS

I have tested this on a couple of machines now but as always

with no guarantee that it will work on yours :)  (Although it should)