TOPSAR Split and Merge with different swaths/bursts

Hi Andy,

As I mentioned complete the two parallel processing’s chains until reaching the unwrapped phases of both results and convert the phase to displacement, then create the mosaic

1 Like

I’m not interested in displacement but absolute elevations. Do you think that would make sense to process two completely distinct DEMs? I’m not sure.

But I didn’t see any refers in your above question that you gonna to create a DEM, Otherwise my answer would be different.

And my answer is I have no idea about your study area and this is important, also in this case to avoid the problematic issues especially in the neighbors area, I’d suggest process the sub swath 4,5,6 and their opposites sub swaths together, Finally aftermath getting your DEM, subset the area of interest you’d like to work in.

sorry, I should have pointed that out. Actually, a nice interferogram (and a coherence image) without phase jumps would be sufficient at first.
My study area is in a very arid region in Mai with low vegetation cover. So actually quite suitable conditions for interferometry.

I now debursted the full scene and made a subset. Takes longer but better handles the sub-swaths.
Thanks for your suggestions and comments, Falah!

excuse me sir
how you extract your study area on the burst and subswath?

with TOPS Split Operator: The Order of DEM Creating Steps

thank you,but it is not clear to understand my area is in 1 subswath or 2 swath
is any exact method to extract it?

I see you had merge problem,can you find a solution for it?
in which step you i should merge images?

you can open the different sub-swaths in SNAP and check visually. Or you open the kmz file in the preview folder and have a look at the area in Google Earth. But it is not geocoded very well.

Hi @ABraun

I also have a region that I need to create a DEM which overlaps swaths IW1 & IW2.
Furthermore, I also wish to process only the subswaths of each swath that contains my ROI
I have read the posts above, however I am still confused on the process.

Could you please check my workflow:

For IW1 and IW2:
(1) Coregistration (TOPSAR split + apply orbit file)
(2) Interferogram formation
(3) Deburst

Once steps (1) to (3) have been completed for IW1 and IW2 then:
(4) TOPS merge?

Sorry, I’m not experienced with analysis of two swaps and am not sure ar which step the merge should occur ar best.
I would test it directly after the TOPS coregistration step. What happens when you apply Merge after debursting as you suggested.

Just a remark on DEM generation. The results are often very bad compared to freely available DEMs like SRTM or AW3D30, despite of the higher spatial resolution, because of temporal decorrelation over vegetation and atmospheric artefacts.

Use image pairs with a temporal baseline as short aa possible and a perpendicular baseline larger than 100 meters. Also, avoid data from rainy days.

1 Like

@ABraun thanks again for the speedy reply and clarification

I’ve kept Btemp to <= 24 days, and 300m>Bperp>=100m However, this is the best I get:

This is what a profile of my DEM vs SRTM looks like: (elevation and distance in metres)

Here is my DEM. I do not understand why it looks like this

This is the coherence:

When you say that the results are often bad, is this what is generally obtained, or should I see if I can get better results?

Thank you

not bad! You already did anything to create a good DEM (baselines, data…), there is no bad trend in your data and the overall elevations seem very reasonable. I just gave these hints because many people expect any SAR data pair to produce flawless DEMs without knowing about the error sources.

I see a few unwrapping errors in the middle left. Maybe you can try unwrapping again with SMOOTH instead of TOPO but I can’t promise that this will solve them. But you can simply copy the exported folder, change the snaphu.conf file directly (open it with a text editor) and run it again.

For a Sentinel-1 DEM this is fine I would say, most others I have seen are way worse. Another successful example: The Order of DEM Creating Steps

1 Like

Thank you @ABraun! All your help is much appreciated :smiley: I will try unwrap export with Smooth.

One last question, please. How does one visually identify unwrapping errors?
Also could please let me know if you are referring to the region within the red circle (below). It is a reservoir:

the errors you highlighted are probably because of low coherence in the valleys (wetlands?).
Unwrapping errors are unnatural phase jumps such as here:

1 Like

@ABraun Ok, I understand. Thanks again.

And yes! It is a valley of sorts - its a mostly empty dam. This was when the dam was at 14-18% capacity.

Hi Dear falahfakhri
How can I process the area inside the two subswath (half inside IW1 and half inside IW2)
Thank you, if you tell me your orders.

So have you figured out at which step should we begin merging two subswaths? I have got the same situation too, my study area lies between two sub-swaths of 2 and 3 bursts of Sentinel 1 SLC images. I am afraid scalloping effect would be an issue

according to the predefined graph in the Graph Builder, TOPS Merge comes at the end of all processing steps.