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In 1999, the US and Japan jointly launched the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) into orbit. One of the goals of the ASTER instrument mission was to conduct detailed terrain mapping of the surface of the earth.

Colorized ASTER GDEM Map of Africa

The ASTER global digital elevation model (GDEM) covers the planet from 83 degrees north to 83 degrees south at 1 arcsecond resolution and is the "most detailed 3d map of the Earth ever made."

In June of 2009, the ASTER GDEM was released to the public.

For us as builders of long distance wireless networks, this is mostly of interest because it allows us to do more accurate computer modeling of radio lines of sight to answer the basic question "If I build antennas at point A and point B...can they talk to each other?"

If you want to try it out for youself, see our detailed technical guide on how to download and convert this data to a format compatible with RadioMobile, the widely used free software for modeling radio propagation.

• Obtaining ASTER GDEM Data and Converting to SRTM1 for Use With RadioMobile

We recommend ASTER GDEM in addition to SRTM3 (previously the highest global terrain model available) as some of our partners in Nepal have had significant difficulties with "voids" or blank spots in the SRTM3 data right in the middle of their project sites.

Using SRTM3, RadioMobile will tell you a line of sight through a void will work, when in the real world it might not. We have seen significantly fewer voids in the ASTER GDEM data. If SRTM3 has voids where you need to plan a link, try the ASTER data.

=Downloading ASTER data from NASA's WIST=

The process for getting the data is relatively easy, and fortunately there is a very good tutorial here:

http://www.echo.nasa.gov/reference/astergdem_tutorial.htm

That walks through the process of using the "NASA Warehouse Inventory Search Tool" to select the ASTER DEM (terrain) data for the area you need.

At the end of the selection process, you will receive email instructions on downloading your data via FTP. I recommend downloading the single zip file containing all your selected ASTER tiles.

(Note that you can only select 1000 ASTER tiles at once for download. If you want to download a larger area you'll need to break your searches into smaller pieces).

=Uncompressing the data=

The zip file contains 2 types of files:

1) ASTGTM_NxxExxx.zip: Zipped data for each ASTER DEM tile
2) ASTGTM_NxxExxx.zip.xml: XML coordinates describing location of each tile.

Delete the xml files (or move them elsewhere), and set up your favorite unzipping utility to unzip _all_ the ASTGTM_NxxExxx.zip files. Each one contains a Readme.pdf file, so you'll need to tell your unzipping program to overwrite those without prompting.

Each ASTER DEM tile contains three files:

1) ASTGTM_NxxExxx_dem.tif: The ASTER terrain data in geotiff format
2) ASTGTM_NxxExxx_num.tif: A "Quality Assessment" file
3) Readme.pdf: Interesting info about ASTER data

Delete or move the *_num.tif files and the Readme.pdf file, so you're left with a directory full of ASTGTM_NxxExxx_dem.tif files.

=Converting to SRTM1=

Because Radio Mobile doesn't speak Geotiff, you'll need to convert these with Roger's batch ASTER conversion tool (called ASTERbatch.zip and available in the files section of this group, http://groups.yahoo.com/group/Radio_Mobile_Deluxe/files/)

The batch utility is very easy to use. Just unzip Asterbatch.zip somewhere and double click AsterBatch.exe.

There's no browse button, so you'll have to manually type the path to your ASTER dem geotiff files from above in "ASTER Path." For "SRTM Path" just type the full path to an existing folder you want the utility to place the converted tiles in.

Click "Proceed" and wait.

Once the conversion is complete you'll be able to use the folder you specified as "SRTM path" just like any other SRTM data in radio mobile.

=Qualifications=

Keep in mind, NASA qualifies the version 1 ASTER GDEM by saying it should be viewed as "experimental or research grade" and acknowledges that it "does contain residual anomalies and artifacts that degrade its overall accuracy."

However, if the quality of the ASTER DEM is good in your project area, it can offer a significant improvement over the 3 arcsecond SRTM3 data.