Image distortions when pre-processing Sentinel-1 SAR using ACE 30 DEM

Microwave Toolbox
#1

Hi,

I am pre-processing Sentinel-1 SAR GRD imagery with IW swath. The pre-processing step includes terrain correction where a DEM needs to be selected. Since my ROI is in Greenland, I use ACE30.

  1. I recently noticed irregular stitched spots in certain regions of the SAR imagery after preprocessing is complete. When I switch to ASTER 1 sec GDEM, this issue does not show up.

Aster1SecvsACE30
Aster1SecvsACE30952Ă—675 473 KB

  1. Another issue is that there is a warping in the shape of certain fjords and mountains when using ACE 30 GDEM.

ACE30Warping
ACE30Warping954Ă—677 450 KB

Can you please clarify on the reasons for such differences? Also, in such case should I restrict to ASTER 1 sec GDEM?

Thanks

#2

What DEM resampling have you selected in the Terrain Correcton step?

#3

ACE30 is a 1km resolution DEM based on altimeter data - looks like it also has large artefacts in some areas.

In the future SNAP will be able to download the free Copernicus DEM that is global, much higher resolution and of very high quality.

1 Like
#4

I also want to add that ASTER is not automatically downloaded, so if you have not manually done it and placed the file in the correct folder (or entered it as an external DEM) probably not terrain correction will be applied and the data is only ellipsoid corrected.

1 Like
#5

HI @ABraun, I use BILINEAR_INTERPOLATION for DEM resampling

#6

Hi @ABraun,
How to know if the terrain correction is applied since there is no exception generated at the end when running with ASTER 1 SEC?

#7

SNAP looks in the DEM folder for correct tiles. If none are found, a flat surface is assumed.
Only the DEMs flagged with “AutoDownload” are automatically retrieved.

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#8

Hi @mengdahl,
Thank you for the information. How to overcome these artefacts in the image then? Do you have any suggestions for the DEM in terrain correction. It cannot be SRTM since I am working in region > 60 deg north.

#9

Oh okay, thanks @ABraun. So then Is there a work around for the large artefacts as shown above in the image 1? Are there any other DEMs I can use suited for this problem?

#10

as I said, you can manually download a DEM of higher resolution than ACE30, for example ASTER or AW3D30, for this area and select it as “external DEM”

grafik

#11

Thanks @ABraun! The reason I am looking for an automatic download DEM as a solution is because I have a large number of tiles spread across different regions that would otherwise require a manual download of DEM.

If I use External DEM download, are there any processing steps that I need to keep in mind for the DEM that are specific to SNAP toolbox. In other words, is there anything to keep in mind when using external DEM in SNAP toolbox for e.g: naming convention, file format constrains?

Also why is the warping seen in above in image 2? Is it dependent on the fact that ACE 30 DEM was applied and in the non-warped image there was no DEM used (as ASTER is not auto downloaded)?

#12

if you have many distributed areas, you can try GETASSE or CDEM (both AutoDownload) and compare if they produce less artefacts than ACE30. But they are also of low resolution compared to SRTM or ASTER.

If you decide to manually dowonload DEMs for terrain correction, you just have to make sure that these are projected in WGS84 and stored as GeoTiff. Please have a look at the help pages regarding this point

grafik

1 Like
#13

Thanks @ABraun, I will try that and see if the artifact issue gets resolved.

#14

There is the GIMP DEM for Greenland, but you will need to import it manually.

https://nsidc.org/data/nsidc-0645

#15

Hi Marcus,

Thats a good idea! However, the GIMP DEM for Greenland is only available up to 2009. Will this still work?