SNAPHU Unwrapping


#266

Hello,

After phase unwrapping, I tired to convert the phase values into height but the resultant values look too strange to me. In this regards, please guide me that:
what do these height values represent?
how to interpret the results?
whether it is height of the object from ellipsoid or it is distance of the satellite from the object or anythingelse?

Work flow: 1. Co-registration, 2. Interferogram Formation & Coherence Estimation, 3. Deburst, 4. Phase Filtering
5. Phase unwrapping, 6. phase to height conversion, 7. Terrain Correction.


(Sentinel-1A, SLC, IW-1, Polarization:VV, Study Area: Karachi City Pakistan)


#267

The height-measurements are relative. If you know the height of a feature you can adjust the values to match that.


#268

if you do not mind, can you explain a little bit.? relative in what sense. I know heights of some sample building, how should i match it. thanks for guidance.


#269

The height-values are relatively correct meaning that the difference in height between two points is “correct”. For example for points A and B:

A = 23222m (from an arbitrary reference)
B= 23327m (from an arbitrary reference)

B - A = 105m (relatively correct height-difference)


#270

Need for phase unwrapping
The discontinuity of the extracted phase appears when an extreme value, or , is reached; the phase then jumps to the other end of the interval, or , even though physically the optical phase is continuous and relatively softly increasing or decreasing. As an example, let us consider a continuous phase proportional to a physical quantity (displacement, thickness, velocity…) varying linearly along a pixel row of the detector.
python


#272

hi
I got this concept but can you tell me, how can i do this in software so all values changes accordingly.
waiting for answer
Thanking you


#273

If I have to see the co-registration accuracy percentage or RMS Error in co-registration then is there any way for that in snap?


#274

open the InSAR stack tool

It will show you the coregistration error results


#275

after following all step for land deformation I got the following results plz tell me
how can I get the accurate values for Elevation as well as Displacement Using the ground surey values of my study area . if possible tell me steps i have to follow in SNAP software for this.
thanks in advance


#276

how do your inteferogram and your coherence look? Your image pair is probably not the most suitable and land cover is hindering proper phase retrieval.


#277

following are my interogram and coherence map


#278

there is a pattern, but quite noisy. Did you apply multi-looking and filtering to the interferogram?


#279

i applied filtering but not multi looking
and yes i did Subset also after Deburst


#280

I don’t think that you can get better information that the freely available SRTM from S1 in this area.


#281

thank you sir
these result showing relative height and displacement
how i will get absolute height and displacement (steps in software)?
I know the concept mentioned above that i have to give value to a point in map
but how ?


#282

technically, you just use the “phase to elevation” module which uses a DEM as auxiliary informaiton to make absolute values. But influence of atmosphere, low quality areas in the interferogram and processing errors during the unwrapping add up to unrealistic results. The only option is to use different image pairs and compare.
The temporal baseline is just too high, I guess.


#283

Images i used were of 14 and 26 jan 2018
So I think temporal baseline is correct ?
but again thank you for such a fast replies
now I will try on different data sets .
one last question you said these are the best possible result for that reason , why ?


#284

yes, only same path acquisitions. INSAR / DINSAR - Perpendicular baseline calculation


#285

Whether the sentinel-1 data is suitable for topographical studies?
Can building / feature height estimation be done by using this data?
Thanks


#286

The spatial resolution of Sentinel-1 is not high enough for building heights.

Here is an example where elevation was successfully retrieved from Sentinel-1: The Order of DEM Creating Steps

But in many other cases, atmospheric conditions, low coherence and unsuitable perpendicular baselines are hindering good results.