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Tutorial: Stream and Catchment Delineation

3. Download DEM Tiles

For the Rur study area, we will download the tiles from the SRTM 1 Arc-Second global data set. Since the end of 2014 a 1-arc second global digital elevation model (approximately 30 meters at the equator) has been released as open data. Most parts of the world have been covered by this dataset ranging from 54 degrees south to 60 degrees north latitude. A description of the SRTM data products can be found here.

The following steps explain how to download the SRTM DEM tiles for the study area using the SRTM-Downloader plugin.

1. Start QGIS Desktop. We’ll start with a new project. In the Browser panel, add the folder with the data for this lesson to the Favorites as you learned in the previous lesson.

2. Drag the boundingbox.shp layer from the Browser panel to the map canvas.

This polygon defines the boundary of our initial analysis. Normally you have to create such a polygon yourself or use the map canvas coordinates. We need this to clip the DEM to the study area. Note that the projection of the project is now set to the projection of the boundingbox layer.

3. Install the SRTM-Downloader plugin from the Plugins Manager.

SRTM Downloader plugin

4. Click the SRTM-Downloader srtm downloader iconbutton in the toolbar.

5. In the SRTM-Downloader dialog, click Set canvas extent. It will fill in the coordinates of your map canvas in GCS coordinates.

6. Choose for Output-Path the folder where you're storing the files for this lesson. Use the  button to locate the folder. Don't keep the default temporary folder! You'll loose the data.


7. Click the Download button.

8. The first time you use this tool, you will see a pop-up that asks for your login. Follow the link to make an account and fill in your credentials in the pop-up. Click the box to Save Credentials and click OK to proceed with the download.

Six SRTM tiles are now downloaded in the SRTM HGT raster format and added to the map canvas.

9. Click OK when the pop-up appears that the download is complete and click Close to close the SRTM-Downloader dialog.

Alternatively, you can download the SRTM tiles from USGS Earth Explorer using the same credentials. You can also download many other free datasets from the site. Another great plugin is the OpenTopography DEM Downloader plugin. The latter can be used to download different DEM products clipped to a specified extent.

You'll see that the plugin has added:
  • A layer called N50E005_N51E007.vrt. This is a virtual raster that has mosaiced the individual tiles.
  • A layer group srtm_images with each SRTM tile as a .hgt raster file.
The advantage of a virtual raster is, that it is not a large raster file, but a small xml file that can be used for intermediate results.

10. Right-click on one of the tiles and choose Properties to review the metadata.

Layer Properties of a Downloaded SRTM Tile

  • What is the projection?
  • What are the map units?
11. Remove the srtm_images layer group, because we'll continue with the mosaic. Right-click on the group name and choose Remove Group... from the context menu. In the popup, click OK to confirm.



Watch this video to check the steps: