SNAP searches non-existent SRTM tiles

Microwave Toolbox
1

Hello

I recently was trying to terrain correct a Sentinel-1 image. The image covers land and water.

The problem seems to be that when SNAP searches for a DEM it expects one must exist. The image I am using (S1A_IW_GRDH_1SDV_20201104T224257_20201104T224326_035105_0418C7_545E) has the north east corner that puts part of the image in the N12W068.SRTMGL1.hgt.zip tile. However, the N12W068.SRTMGL1.hgt.zip does not exist since it is all water. However, the s/w repeatedly looks for the DEM and tries to unzip it:

SEVERE: org.esa.snap.core.dataop.dem.ElevationFile: error in opening zip file
INFO: org.esa.snap.core.dataop.dem.ElevationFile: http retrieving http://step.esa.int/auxdata/dem/SRTMGL1/N12W068.SRTMGL1.hgt.zip
SEVERE: org.esa.snap.core.dataop.dem.ElevationFile: error in opening zip file
INFO: org.esa.snap.core.dataop.dem.ElevationFile: http retrieving http://step.esa.int/auxdata/dem/SRTMGL1/N12W068.SRTMGL1.hgt.zip
etc…

Its seems that SNAP does nto understand that the absence of a tile must indicate the cell is all water, I think.

All the best,
Pablo

2

indeed, this tile toes not exist. I modified the title for the sake of readability.

grafik
grafik1022Ă—587 143 KB

I wonder why SNAP wants to access this tile.
What data are you processing and which step causes this error message?

3

so SNAP wants to access a SRTM tile which does not exist.
You could create a raster (constant 0) in the same format and spatial resolution and place it inside the aux folder so SNAP takes this instead.

4

Hello ABraun

any tips on on how to create this file? Because I am not sure how to create a file in the SRTMHGT/SRTMHGT File Format.

Thanks,
Paul

5

I would do this in SAGA.

  1. open one of the hgts to check their spatial resolution (in degrees) and the data type (int/float…)
  2. identify the corner coordinates of the tile missing tile
  3. use the “Constant grid” tool to create a raster with the extent, resolution and file type as desired (first image)
  4. Export it as ESRI labelled HGT (second image) and put it into the aux directory to the others (use the file name given in the error message)

grafik
grafik1188Ă—720 11.2 KB

grafik
grafik1188Ă—720 29 KB

1 Like
6

Great - thank you. I will give this a try. I have SAGA downloaded but I have never used it, so you are helping me to expand my horizons. :slight_smile:

7

@ABraun

Once I got the correct version of SAGA, I was able to fake the DEMs. Running SNAP now and it seems to be happy. Thanks for all your help.

Regards,
Paul

8

great to hear, good job!
I moved the posts back into a new topic because it is different from the download errors in the other topic.

9

Yes, it seemed that there were two different topics being discussed. So it was good you separated the two threads.

But I thought I would let you know that the DEM download in SNAP produces an empty zip file for non-existent tiles. So, that is why the error shows up as a zipfile problem which obscures the fact that no tile exists. Not sure if that is the intended behavior.

Regards,
Paul

1 Like
10

thank you for reporting. Seems like the SRTM download is faulty in general at the moment

11

Hello,

I am seeing the same issue with Copernicus 30 m DEMs. All water cell creates an empty file tif file.