Terrain correction problems

Vileda · January 30, 2020, 10:04am

Good morning everyone,
I’m processing some interferograms from Sentinel 1 images but I’m facing some problems at the end of the processing chain with the Terrain Correction (Range-Doppler Terrain Correction) tool.
The problem is that sometimes (not always and I can’t understand why) my interferogram is cut passing from radar to geo coordinates.

Has anyone of you run into the same problem? I tried using different DEMs, but the problem still persists.

Thanks four your help!

ABraun · January 30, 2020, 11:35am

if your study area contains ocean (or inland water) areas, this option crops them out in the resulting product:

Vileda · January 30, 2020, 1:38pm

Thanks, @ABraun! my area is in the middle of the Alps, no water close to it… however, I tried without that option, but I still get the same results.
I’m disappointed because It’s not a constant error. Sometimes I can get the entire scene well reprojected, but other times,for unknown reasons, it doesn’t work and cut part of it.

Thanks for your time and help!

ABraun · January 30, 2020, 1:47pm

then this line is not because data is left out but because pixels are moved to their correct position. The geometric distortions are corrected in the image, causing one (or both) border in range direction to be adjusted to the terrain topography.
This is completely normal and should actually happen for every image in this region. The stronger the topography, the stronger this effect.

Have a look at this tutorial: http://step.esa.int/docs/tutorials/S1TBX%20SAR%20Basics%20Tutorial.pdf
The same effect occurs there after terrain correction.

Vileda · January 31, 2020, 3:35pm

Ok thanks @ABraun.
So it’s just a “visualization” problem… thanks for your help.

Vileda

ABraun · January 31, 2020, 5:01pm

let’s say it is simply a higher “product level”

image source

hriston_bg · February 7, 2020, 2:02pm

@ABraun Thank you for the clear picture - it explains a lot.

rahepu · September 13, 2024, 1:58pm

I am having the same issue, SNAP seems to be cropping the raster in a way it should not. Data is missing.

rahepu · March 31, 2025, 7:01pm

Has anyone found a way to work around this SNAP TC issue? I am aware it does not always happen, so I don’t know exactly how to replicate it universally.

Edit: I tried downloading the SRTM and using it as an external DEM, ensuring that the area is larger than the SAR image, but it still produces the same error, so whatever causes the edge cut off is probably does not appear the SRTM data being missing, but rather the boundaries of the corrected raster being calculated incorrectly, somehow.

Edit 2: It also happens with the ellipsoid terrain correction, but not with the geolocation grid.

For reference, here is the same DEM terrain-corrected using SARscape, it does not cut off the edges.