Why to Use ASTER Data
There are two main reasons to think about going through this process to use ASTER data instead of SRTM3 for RadioMobile wireless link planning: higher resolution, and voids.
The higher resolution ASTER sensors made the creation of a 1 arcsecond resolution global digital elevation model (GDEM) possible. This is significantly better than the 3 arcsecond resolution offered by SRTM3 (previously the highest global terrain model available.).
See below for a comparison of RadioMobile terrain images comparing the two.
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. 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
The ASTER GDEM data is available through the the NASA “Warehouse Inventory Search Tool” or WIST.
The process for getting the data is relatively easy, and fortunately there is a very good tutorial available.
Basically, you create a WIST account for yourself, specify the region you want to download ASTER data for, go through an “ordering process” (data is all free), and submit your order.
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 offered, rather than downloading ASTER tiles individually.
Note: You can only select 1000 ASTER tiles for each WIST order. If you want to download all the data for a larger area you’ll need to break your search into smaller pieces.
Uncompressing the data
Your downloaded zip file contains 2 types of files:
- ASTGTM_NxxExxx.zip: Zipped data for each ASTER DEM tile
- 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:
- ASTGTM_NxxExxx_dem.tif: The ASTER terrain data in geotiff format
- ASTGTM_NxxExxx_num.tif: A “Quality Assessment” file
- 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.
Note: I found this process much easier with a command line unzipping program than I would have with an unzip gui.
Update: To use the newly available ASTERv2 datasets with the existing conversion utilities, you’ll want to rename the ASTERv2 files from ASTGTM2_NXXEXXX_dem.tif to ASTGTM_NXXEXXX_dem.tif
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. (Right click the link and “save file as” to download the utility to your desktop.)
The batch utility is very easy to use. Just unzip ASTERbatch.zip somewhere and double click ASTERbatch.exe.
The utility doesn’t have a browse button, so you’ll have to manually fill in “ASTER Path” with the full path to wherever you stored the ASTGTM_NxxExxx_dem.tif files above.
For “SRTM Path” type the full path to an existing folder where you want the utility to place the converted files.
Click “Proceed” and wait. If you are converting a large number of ASTER tiles, this can take a long time.
Once the conversion is complete the folder you specified for “SRTM path” will be full of SRTM1 tiles ready for use by RadioMobile.
Warning
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.”
We are still experimenting with the converted ASTER data for our RadioMobile work, but we’re having mostly good results.
A big advantage of the ASTER data is that it seems to have far fewer “voids” in areas where the SRTM3 data had many. These voids in the SRTM3 data showed up as rectangular areas with no terrain. The voids were particularly irritating when they showed up directly in the line of sight between two project sites.
Some of our Nepali partners have had problems with these voids when planning wireless projects, and this ASTER data could provide them with a possible solution.
The version 1 ASTER GDEM data definitely contains some strange artifacts, though. In one case a paved road in the flatlands of Botswana showed up as a 100m deep trench in the ASTER data. Not so great.
Hopefully, we will soon see a version 2 of the ASTER GDEM that will address some of these issues. In the meantime, 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. If SRTM3 showed significant voids in your area, you’ll definitely want to give ASTER a shot.
Good evening, Andris –
Thank you for the great post that helped walk me through getting the ASTER data. I think I did everything you mention, when I run ASTER2SRTM, point the ASTER path to where the ASTER tile .tif file is (only one tile), and put in my destination folder, I hit Proceed, and within a second or two there’s a nice DEM map in the ASTER2SRTM display box. However, there’s no file saved to the directory I have specified. I’ve even tried using a number of different directories for destination, but no luck. Any ideas?
Cheers – Jon
Jon Adams
It seems I skipped one vital point, to rename the file from
ASTGTM2_N24E121_dem.tif
to
ASTGTM_N24E121_dem.tif
When I did that, all worked well.
Thanks again!
Sincerely, Jon
Jon Adams