How to Import SRTM Terrain from NASA's Earthdata into STK

High-resolution terrain data is paramount to accurate analyses involving platforms operating in close proximity to the ground, whether it is a satellite ground station, driving vehicle, or a low-flying aircraft. Terrain can cause line-of-sight obstructions or even ground wave reflections with radio signals. STK has the capability to integrate custom terrain into its TIREM model to analyze beyond-line-of-sight RF propagation over land and water. Thankfully, NASA's Earthdata service publishes 30 m resolution terrain, free of charge, for a huge swath of the planet's surface. The data were collected as part of the Shuttle Radar Topography Mission (SRTM) are available on line in 1 deg x 1 deg tiles over Earth's middle latitudes. Here's how to convert that data into a format readable by STK:

1.  Download and install QGIS, a free and open-source GIS project. While these instructions are specific to QGIS, you can use a similar workflow in the GIS application of your choice.
2. Create an account on NASA's Earthdata service. Search for "NASA Shuttle Radar Topography Mission Global 1 arc second V003" or by data code SRTMGL1. These data are published in a custom *.hgt format, and you can view the dataset documentation for more information. Find and select the geographic region of your choice. When pulling the data down, you can choose to download each tile individually or use the bash script generated by Earthdata to automate the download of large numbers of terrain files.
3. Unzip the downloaded terrain files and import them into QGIS. After extraction, you can drag and drop the *.hgt files directly into your application window to import them automatically. QGIS will recognize the data format and display it automatically. If you have two or more tiles, you may notice that each tile has a different color scheme. Don't worry, each tile has its own maximum and minimum elevation, and the color scheme is bound to those maximums and minimums. Also, all seawater has an elevation of 0. This is important for STK's TIREM capability, since this will properly trigger the seawater propagation calculation for large swaths of ocean. Other bodies of water, like lakes and rivers, have their elevation tagged as a nonzero value.
4. In the top toolbar, select Raster -> Miscellaneous -> Merge. Take all of these files and merge them into a single cohesive file. Select all of the *.hgt files in the input layers section and select any save directory of your choice. Be sure to export as the default *.tif file, since STK can read this format in natively. Click Run to merge the files. This may take some time, especially for large numbers of tiles. Any gaps will be filled with a 0 elevation value such that the output GeoTiff file is rectangular. When it is complete, QGIS will display the merged dataset for inspection. Notice how the terrain coloring is no longer segmented.
5. You can now load the merged dataset into STK for analysis. Open STK's scenario properties, go to the Terrain page, and click Add to select the output *.tif file. Since the SRTM dataset is referenced to the WGS84 ellipsoid, be sure to select Tagged Image File Format (tif) in the file type selection. Do not select MSL, or the terrain may not load in as expected. If desired, turn off the Terrain Server as well.

That's it! Your custom SRTM terrain is now available for analysis in STK. If you would like to visualize your terrain in the 3D Graphics or 2D Graphics window, check out AGI's Imagery and Terrain Converter help page or follow the steps in this tutorial. If you're looking for terrain within the Continental United States, the USGS National Elevation Dataset contains 1/3 arcsec terrain (approximately 10 m). Check out this tech tip for getting that data loaded.