Vertical displacement computation, which angle can be assumed as more adequate?

Good morning,
I would like to have advices on this doubts of mine on which angle is better to be used when considering the theta.
I read a lot (here on the forum) on the Incident Angles I can retrieve by exporting from the Range-Doppler Terrain Correction.
My case study is located in a flat plain terrain that was “reclamated” many many decades ago and suffered subsidence. The main aim of the research is to evaluate if the phenomenon is still strong as before or if the solution applied in the last decade in the area worked effectively.

From the RDT Correction, using the SRTM 1 Sec HGT (autodownload) I exported the LocalIncidentAngle (LIA) to be used in the mathematical formula
(if my assumption is wrong please let me know), but in the areas in which are present the farms structures(owners’ houses, etc.), also in the nearby ground, the pixels values were 0, so consequently I cannot use the LIA. The choice behind using the LIA is related to the fact that I thought it was more correct than using a medium IncidentAngle for the flat plain area (no slopes involved in the study area).

So, thinking that was maybe the image causing the value 0, I changed image and exported again the LIA, but the pixel values 0 are still there.

I was wondering, in these cases of value 0 for the LIA in pixels, is it correct to assume the use of the Ellipsoid Incident Angle (EIA) from the moment that my area is flat?

I also carried out the decmposition by using QGis, but I would like to have the “1D” decomposition for a pair of PSs (nearby a HP Leveling benchmark).

Please let me know if any or all my assumptions are incorrect, I am eager to learn and improve!

Thanks in advance!