Downloading Shuttle Radar Topography Mission (SRTM) data using QGIS
One of the key components of geospatial analysis is digital elevation model (DEM) data, which provides information about the height and slope of the earth’s surface.
The Shuttle Radar Topography Mission (SRTM) is a NASA mission that has provided high-resolution global DEM data, and with the right tools, it is possible to download and use this data for free.
Further reading: the difference between DEM, DTM, and DSM
In this blog post, we will guide you through the process of downloading SRTM DEM data using the popular open-source software QGIS.
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Installing the SRTM Downloader Plugin
To get started, you will need to install the SRTM Downloader plugin. Here’s how:
- Open your QGIS workspace.
- Navigate to Plugins in the menu and select Manage and Install Plugins.
- Search for the SRTM Downloader plugin, and click on it to install.
Once installed, you will find the SRTM Downloader listed under the Plugins menu.
Setting Up a Base Map
To effectively specify the boundaries for downloading your SRTM data, it is helpful to have a base map. You can achieve this by installing another plugin called Quick Map Services.
- Go to Plugins and search for Quick Map Services.
- Install the plugin.
After installation, access the Quick Map Services under the Web menu. Select OSM (OpenStreetMap) to add the base map to your workspace.
Changing the Coordinate Reference System
Once the OpenStreetMap is displayed, it’s crucial to set the correct coordinate reference system (CRS). Follow these steps:
- Go to the Coordinate Reference System settings.
- Change it to EPSG 4326, which is the WGS 1984 geographic coordinate system.
Zooming into Your Area of Interest
Now, zoom into the area for which you want to download the DEMs. For this tutorial, we will focus on the southern part of Italy.
Refining Your Extent and Downloading Data
After zooming into your area of interest, refine your extent using the zoom tool to draw a rectangle around the area you want. Ensure not to change the extent after setting it.
Next, open the SRTM Downloader plugin and click to automatically extract the boundaries of your specified extent. Make sure to tick the option to load images into QGIS.
Finally, click on Download. If prompted, enter your NASA Earthdata username and password, which you can create if this is your first time using the service.
Handling the Downloaded Tiles
Once downloaded, you will see the tiles that cover your specified area. If you wish to merge these tiles into a single raster file, navigate to:
- Raster > Miscellaneous > Merge.
- Select the downloaded tiles and click Run.
Exporting Your DEM
To save your digital elevation model, right-click on the merged raster file, go to Export, and select Save As. Choose the GeoTiff format and specify a file name.
FAQ
What is SRTM data?
SRTM data provides global elevation data and is widely used for geographic analysis and modeling.
Do I need to create a NASA Earthdata account every time?
No, you only need to create an account the first time. Subsequent downloads can use the same username and password.
Can I download DEM data for any location?
Yes, as long as the area is covered by SRTM data, you can download it for any location globally.
What if I encounter issues during the download?
If you experience any issues, ensure your username and password are correct and that you have a stable internet connection.
That’s it! You have successfully downloaded SRTM digital elevation model data using QGIS.
Shuttle Radar Topography Mission (SRTM) and the data you just downloaded
The Shuttle Radar Topography Mission (SRTM) was a collaborative project between the National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA), with participation from the German Aerospace Center (DLR) and the Italian Space Agency (ASI). The primary goal of the mission was to create high-resolution, near-global digital elevation models (DEMs) of Earth’s land surface.
The mission was carried out from February 11 to February 22, 2000, using the space shuttle Endeavour on its STS-99 mission. The SRTM payload consisted of a specially modified radar system known as C-band Synthetic Aperture Radar (C-SAR) and an X-band Synthetic Aperture Radar (X-SAR). The C-SAR was operated by NASA, while the X-SAR was a joint effort between DLR and ASI.
The radar systems functioned by sending out microwave signals that bounced off the Earth’s surface and were then received by the shuttle’s antennas. The differences in the time it took for the radar signal to return, as well as the phase difference between the transmitted and received signals, were used to calculate the elevation of the Earth’s surface.
The SRTM utilized a technique called interferometry, in which two radar antennas were mounted on the shuttle, one at the end of a 60-meter (200-foot) mast and the other on the shuttle itself. The large baseline between the two antennas allowed the system to measure height differences with great precision, resulting in high-resolution elevation data.
The mission covered the majority of Earth’s land surface between 60 degrees North latitude and 56 degrees South latitude, which includes about 80% of the Earth’s total landmass. This coverage encompasses most of the world’s inhabited areas and major landforms. The elevation data collected by the SRTM was processed into DEMs with a horizontal resolution of 1 arc-second (approximately 30 meters) for the United States and 3 arc-seconds (approximately 90 meters) for the rest of the world.
