External DEM in offline mode

Microwave Toolbox
1

I am attempting to automate the processing of Sentinel-1 data using SNAP’s ‘gpt’ on high-performance computing facilities. I’ve run into the main issue that the nodes used to execute the jobs do not have access to the internet, and thus, cannot automatically download the DEM files required for the data processing.

Various previous posts have alluded to this issue but without providing a solution. So this post is to try and work out where the developments are at currently regarding this issue.

My problem would be easily fixed by simply pre-downloading the DEM .zip files to the .snap/auxdata/dem folder, and then letting ‘gpt’ access these local files during the processing. This approach doesn’t work however as the .xml graph parameters expect an ‘externalDEMFile’ rather than an ‘externalDEMFolder’. As indicated in this post, using a folder for an external DEM was on the developers’ to-do list as at Jan. 2017. So Question nr. 1 is: has there been any progress regarding this feature over the last 1.5 year or so?

In addition, this post indicates that even if the DEM tiles are pre-downloaded, ‘gpt’ will apparently still attempt to connect to the tile server to checksum the local files (thereby sending it into an infinite loop of error messages if the connection fails). This would obviously make the solution of pre-downloading DEM data rather useless. So Question nr. 2 is: Can someone confirm that processing the S1 data with pre-downloaded DEM tiles still requires a connection to the server, and can this behaviour be overridden?

Thanks in advance for any info / help!

6 Likes
2

Exactly same isseu I am facing but yet no solution I got. If by chance anyone got the solution kindly revert back so that it can be helpful.

Thankyou in advance.

3

Hi,

I am also facing this issue. I have downloaded STRM 3SEC (which is a folder filled with zip files) and put it in the auxdata/dem folder. GPT still tries to download it from server, which blocks the process when working offline.

Thanks a lot for any help!

4

did you place them in this directory? \.snap\auxdata\dem\SRTM 3Sec

The files should be named like this: srtm_37_02.zip, srtm_36_02.zip,…

5

Yes, I am pretty sure it is the right folder because when I delete it, it gets automatically created by snap when I relaunch my graph. I indeed have this name format.

Actually, I noticed that I get this error before the attempted download of the strm : “Unable to download http://step.esa.int/auxdata/dem/egm96/ to null”. Could it play a role in not retrieving the dem files?

My dem/egm96 folder is indeed empty. But I got it running on Snap v6.0 with the exact same auxdata/dem folder, and not on Snap v7.0

6

I added the files manually to .snap/auxdata/, this step was looking for and then it worked.
For example:
cp dem/SRTM\ 3Sec/srtm* ~/.snap/auxdata/dem/SRTM\ 3Sec/
which copied:
srtm_21_04.zip
srtm_22_04.zip
srtm_63_17.zip
cp dem/ww15mgh_b.zip ~/.snap/auxdata/dem/egm96/

1 Like
7

Hi,

Is there now a way to specify an external DEM folder in the new release v8 of SNAP ?

I’m trying to set all the SRTM data .tif in a folder and tell snap to used it.

Thanks

8

you mean that you only name a folder and SNAP selects the ones which cover the input image?

9

Yes, exactly.

10

I’m afraid only single files can be used as inputs by now.

11

Thanks for the answer.

And do you know if it is possible to use SRTM 90m instead of SRTM 30m data ?

12

Any DEM which is stored as GeoTiff in WGS84 can be used as external DEM.