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Create Accurate WiFi Links, Free with ASTER GDEM

By Andris Bjornson on December 15, 2009

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.

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.

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.

Filed Under: Connectivity
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Since graduating from Northwestern University with a Physics degree, I have helped build long-distance nonprofit WiFi networks as a volunteer in Nepal, managed communications-hardware deployments for the U.S. Department of State, created a high-volume image archive system for an A-list advertising photographer, and helped tell the story of landmine survivors through documentary multimedia. This multi-disciplinary career path has been my attempt to blend passions for technology, creativity, and global involvement. Outside of work, I am an avid photographer and I try to spend as much time as possible getting to the top of tall things by boot, bike, climbing harness, or ice axe.
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7 Comments to “Create Accurate WiFi Links, Free with ASTER GDEM”

  1. Carlos says:
    December 14, 2009 at 1:43 pm

    Thank you very much for the info. At EHAS Foundation we have been working with SRTM3 for a very long time when designing our long distance networks in the Peruvian jungle. Although the results are passable, in some occasions we also find voids that hinder our simulations. We’ll have a look at the ASTER GDEM to compare and tell you back with our results.

  2. andris says:
    December 14, 2009 at 11:37 pm

    Thanks for reading Carlos, please let us know how the ASTER data works for you. Since it’s such a new dataset, I’m hoping to get a consensus from people around the world using it for wifi planning to see if they see significant improvement in their models.

  3. sebastian buettrich says:
    December 20, 2009 at 7:30 pm

    thanks –

    definitely gonna try it out,

    have worked with srtm3 for a lomg time, for african and asian projects.
    some voids, some areas where the data just isnt good enough.

    eager to see how good aster is.

    cheers,

    s.

  4. andris says:
    December 20, 2009 at 7:42 pm

    Hi Sebastian,

    Great…thanks for reading! Since writing the article, I’ve used the ASTER data to look at links for two different projects in Kenya and things are looking good.

    Any “artifacts” like those I mentioned in the technical howto are fairly easy to spot (i.e. far higher or lower than any surrounding terrain). For the two Kenyan project areas I saw none.

    Converted ASTER data for all of Africa takes up about 72 GB. I know that RadioMobile can read SRTM3 data where each of the individual tiles is zipped. I want to try this for the converted ASTER data to see if I can save a little space.

    Please let us know how ASTER works for you!

  5. Jørn Howlid says:
    November 3, 2011 at 8:35 am

    Hi,

    one year ago I downloaded and converted GDEM.TIF files with the tool as described in:

    http://www.ictworks.org/obtaining-and-converting-aster-gdem-data-srtm-radio-mobile :

    “Converting to SRTM1

    Because Radio Mobile doesn’t understand geotiff files, you’ll need to download this Windows utility to batch convert ASTER tiles to SRTM1 tiles.”

    I used OziExplorer 3D with great success.

    However; As NASA Oct 17 announced ASTER V2, this converter does not convert anymore.

    Can anybody advice me how to convert from .dem.tif files to .hgt files ?

    BR

    -Jørn Howlid
    Norway

  6. andris says:
    November 3, 2011 at 7:02 pm

    Hello Jorn,

    I’m glad you found the ASTERv1 article helpful.

    I personally have not yet tried ASTERv2 in RadioMobile…but I should!

    For very technical questions like this, I always visit the RadioMobile Yahoo Group. They always have great answers.

    http://groups.yahoo.com/group/Radio_Mobile_Deluxe/

    This time was no exception. Their answer:

    “Rename ASTGTM2_NXXEXXX_dem.tif to ASTGTM_NXXEXXX_dem.tif” and the conversion utilities should work.

    I have not tested this myself, but please do report back and let us know if this works for you! I’ll add the information about ASTERv2 to the ASTER tutorial.

    Thanks, and best of luck

    Andris

  7. Jørn Howlid says:
    November 3, 2011 at 10:43 pm

    Hi Andris,

    it worked perfectly! Now I have 1 arc second (30 meters) in Ozi 3D; Excellent 3D!

    Thanks!!
    -Jørn