aah I see! not this is the bad interferogram, that I will not use, derived from DEM-Assisted coregistration and low degree polynomial order in interferogram creation.

You are right, here follows the same image as before, with the same color coding of the elevation DEM

Again the order is the same,

- elevation_DEM
- elevation_add
- elevation_ifg

Now they are! in the previous post,

So, any ideas what to make of it?

Would that make the end displacement map not-trustworthy?

that looks better. Indeed, smaller differences of the used DEM can occur depending on the resampling and pixel size of the inputs. So the variations you see here are totally okay, because the main pattern of displacement will still be derived from the phase difference of the two input products. Topographic phase is just removed to prevent unwrapping errors, so it does not harm if it is differently estimated for the inputs.

OK I see! So after all I can trust the end result? -Would I have to apply any shift to the end result so as to make it match its true ground position considering the elevation_DEM?

I will play a little with the interferogram parameters and post the results again to discuss them, so I can understand how to evaluate them

you can check if there are preprocessed interferograms of your area in this portal: https://comet.nerc.ac.uk/COMET-LiCS-portal/

They are a good reference regarding how good ones look like. Click on a tile, then “Link” then “interferograms” and select a date. There are png files for comparison.

The differences regarding the DEM do not cause shifts, but you have to level all displacements at one stable point. We had a discussion on this here: Different results in the study of land subsidence using the Sentinel-1 satellite image

Sadly there are no available ifgs for my study area, in that site.

Here follows an image of ifg generation:

the ordering is top row left to right and then bottom row left to right (1 2 3 4 5)

- Polynomial Degree 6

Orbit Degree 1 - Polynomial Degree 6

Orbit Degree 2 - Polynomial Degree 6

Orbit Degree 3 - Polynomial Degree 6

Orbit Degree 4 - Polynomial Degree 6

Orbit Degree 5

I have also tried these combinations with varying Polynomial degree from 2 to 8 taking even number, resulting in highly similar ifgs (if not identical).

By the looks of 4 and 5 it seems that the ionospheric effect is eliminated.

-Is it true?

I then chose to proceed with 4, so as not to take the extreme values of combinations.

Then i performed the goldstein filtering and phase unwrapping as shown:

left: Wrapped right:Unwrapped

I am impressed on how similar these two are.

i do not know if that is expected.

Next follows the displacement map, based on the unw/ ifg.:

Left: coherence estimation, right:Displacement (for coh>0.5)

What do you think of the process so far?

Am I lead astray by seemingly “good” results? or are they actually useful results?

I think I got what you said here, i will take this to the big scale. I will also take two consecutive pair this time, based on the approach mentioned above. Subtract the relative difference and see what happens.

I am thinking of using the Polynomial Degree 5 and Orbit Degree 4 combination as being the closest match to the default values of snap, while eliminating those slow varying waves.

After all, i would see no relative displacement in rivers and in urban like areas, right?

just trying to imagine how to assess the end result

PS. Do you think it would be best to coregister non-calibrated products?

Can’t tell which areas are suitable as stable reference point. Once you identified one, make sure it’s value is zero in all pairs for best comparability.

What do you mean with ‘uncalibrated products’?

I mean Radiometric Calibration (or even Speckle filtering).

So far I have been using non-processed products

these operators are applied on backscatter intensity while interferometric methods work with the phase information. So calibration and filtering is not necessary (or useful) here.

OK great! Thank you!

I created 2 interferograms, based on the same pair as always.

They look like this:

Left: Polynomial Degree 5, Orbit Degree 4

Right: Polynomial Degree 6, Orbit Degree 5

Do you think they still suffer from ionospheric affects?

-Are they “good enough” to continue processing?

Based on your experience that is

I really can’t tell from looking at them, sorry.

At least they do not contain these regular stripes any more.

sure! ok, I will proceed and hope for the best!

A funny thing happened,

The SNAPHU plug-in, though it can run a full scene when the ifg is derived from products that are radiometrically calibrated, it produces memory errors when the original products are not radio/y calibrated.

Any insights?

Would it be erroneous to generate ifgs from products that are radiometrically calibrated?

-considering that you entioned: “these operators are applied on backscatter intensity while interferometric methods work with the phase information. So calibration and filtering is not necessary (or useful) here.”

you can’t retrieve inteferograms from Sigma0, so I probably don’t get the point here

Complex calibration would make only sense to me for polarimetric analyses…

yes, you are right. certainly I meant complex r. calibration.

I can’t think of a reason why it fails without complex calibration, but in the end, it will not harm your analysis if you keep it in the workflow

initially i thought it would be that the calibrated product occupied less space, but a simple check-sum revealed that both different versions of the product have the same data in MB. So, i have no idea why this happens