External DEMs

I understand that many people have problems incorporating external DEMs into SNAP (particularly the S-1 toolbox) and that this discussion has been taken up several times. Based on my recent experience, I understand that this problem persists through version 2.0.2.

My area of work is in the northern part of Scandinavia. The SRTM that is automatically downloaded does not extend above 60N. The GETASSE30 does extend north of 60N but the resolution is very coarse. My results have strange artifacts dotting the scene.

One suggestion was to manually download the Aster DEM and copy it into the …snap\auxdata\dem\ASTER 1sec GDEM folder. I’ve done this. But the results are the same as with the external DEM. That is, when trying to co-register a master and slave image, the slave image is completely NaN.

With this in mind, I have a few questions:

  1. Has anyone successfully used an external DEM in any of the interferometry analyses?
  2. If yes, could you please give me detailed information concerning the exact format, file structure, projection, etc?
  3. Does the file(s) in the ASTER 1sec GDEM folder need to follow a certain naming convention or format? I downloaded 20+ tiles from NASA and the result was a single TIFF-file with a strange name.

All help is greatly appreciated!
Michael

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If you’ve download the ASTER tiles, placed them in the right folder and they follow the naming convention ASTGTM2_N00E111.zip then you don’t need to use them as an external dem file. You should select ASTER from the DEM pull down menu.
It’s possible the file naming is not correct and SNAP is not recognizing it. In which case the software will not know the difference than if a tile doesn’t exists.

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We try to use an external DEM in ENVI format to substract the topographic contribution from an interferogram. After many trials we did not success to compute a correct interferogram. Do you know if it is exist some documentation that clearly explain the format expected by SNAP ?
Thank you for helping us

Hi,

I would like to ask what folder I am supposed to put downloaded SRTM DEMs (more of them) so I do not have to download SRTM DEM every time I do terrain correction (I use docker containers and it is downloading SRTM for same location all over so I would like to put downloaded DEMs to docker image with SNAP ).

In the Terrain Correction tool help is: “The tiles will be downloaded to the folder .snap\AuxData\DEMs\SRTM_DEM\tiff. The .snap folder is located in your user folder”. But I did not find such folder in my Ubuntu.

I have two question:
What is the correct path in Ubuntu ?
This is the SRTM DEM which is automatically downloaded http://srtm.csi.cgiar.org/ ?

Thanks

under Linux, you find your DEMs under:
user/.snap/auxdata/dem/SRTM 1Sec Grid/, for example, where user stands for your profile. SNAP automatically puts DEMs there, so if it was used once it shouldn’t be required to download it again.

The folder is hidden however, you can access it with your terminal (maybe that also requires sudo)

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$HOME/.snap or ~/.snap or (often, but not always) /home/$USER/.snap