Terrain Correction in Multi-Temporal Dual-Pol S1 Stack Masks Out Water Despite <nodataValueAtSea> Set to False

In the final step of a multi-temporal dual-pol GRD Sentinel-1 processing graph, we apply terrain correction to a speckle-filtered stack. To retain water bodies, we uncheck the “Mask out areas without elevation” option (in GPT this corresponds to setting to false).

This setting appears to be respected only for the master image pair. For all other images in the stack, water bodies are still masked out (set to no-data).

This same behavior is observed in SNAP versions 9 through 12, both using the GUI tool and GPT, and on Windows and Linux systems.

Has anyone encountered this issue? Is there a known workaround, other than subsetting the stack by bands (dates) and applying terrain correction pair-by-pair?

Any suggestions would be appreciated.

@jun_lu can you help?

A Jira ticket (Jira) has been created to track the issue. We will look into it. Thank you.

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Could you provide more details (e.g. product names, processing steps, parameters used and graph etc.) regarding your processing so that we can reproduce the issue on our side? Thank you.

Hi @jun_lu , of course.

Here is a simplified example using the following two frames:

S1A_IW_GRDH_1SDV_20180725T001325_20180725T001350_022943_027D53_C536.SAFE
S1A_IW_GRDH_1SDV_20180713T001324_20180713T001349_022768_0277CF_7D48.SAFE

The processing is divided into two graphs, one single date and the other one multi-temporal. The only parameters modified from the defaults are the DEM used for coregistration (COP_GLO30), the speckle filter (Refined Lee) and the water mask flag.

single_date_graph.xml (2.8 KB)
multi-temporal_graph.xml (5.5 KB)

I think the problem was caused by that you had the “Mask out areas with no elevation” option selected in DEM-Assisted-Coregistration (see image below). Uncheck this box, the result should be fine.
image

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Dear @jun_lu , as you can see from the multi-temporal_graph.xml file I shared, the nodataValueAtSea parameter, which corresponds to the flag you are pointing out in the GUI, is set to false. Unchecking the flag will not apply masking only to the master in the stack, as you can see below.

Running the example graphs that I provided on those two frames will yield an unmasked master:
unmasked_master

and a masked slave:
masked_slave

this is independent from the size of the stack.

Did you select the “Mask out areas with no elevation” option in DEM-Assisted-Coregistration?

Sorry, I misread. I will check right away if this fixes the issue.

Unchecking the option in the DEM-assisted Coregistration fixed the issue @jun_lu , thank you. We did not notice that option.

I have a question though, is the output master not masked for no elevation by default after coregistration? The help seems to be pretty outdated: Online Help – STEP