Why my DEM made by S1A data have no elevation value?

xydf_1992 · January 1, 2019, 1:03pm

After Deburst, the result is like this: all the district is purple and I can’t see any valueable information.

I suspect that there is a problem with the DEM used by Back Geocoding when coregister

xydf_1992 · January 1, 2019, 1:25pm

After Coregister, there is nothing in Slave Intensity Image, is it normal?
Thank you very much.

ABraun · January 1, 2019, 2:45pm

if your data is simply purple, there must be an error before the interferogram generation step already.

xydf_1992 · January 2, 2019, 5:17am

yes, after coregister, the slave image have nothing. Could you tell me what’s wrong with the slave image, I tried to do this on another computer, but it doesn’t work.

xydf_1992 · January 2, 2019, 6:48am

I tried many times, every time after my S1 TOPS Coregistration, the Intensity_IW_slv image presents black.
No matter how I replace SAR data, the result is slave image black.

ABraun · January 2, 2019, 7:26am

please name the products you use for image 1 and image 2 of the TOPS Coregistration

xydf_1992 · January 2, 2019, 7:36am

S1A_IW_SLC__1SDV_20180907T100450_20180907T100517_023591_0291E9_13F0
S1A_IW_SLC__1SDV_20180919T100451_20180919T100518_023766_02978B_4C8C
S1A_IW_SLC__1SDV_20181001T100451_20181001T100518_023941_029D3D_1192
all these, when it use as master image is ok, but the slave image is always black.

ABraun · January 2, 2019, 10:07am

I made a test with the first two scenes.
I used S1 TOPS Coregistration with ESD

Subswath IW3
VV Polarization
Bursts 1-3
SRTM 1Sec (AutoDownload) for the BackGeocoding

and left all other parameters as standard.

Worked out well.

xydf_1992 · January 2, 2019, 10:34am

I tried S1 TOPS Coregistration with ESD，but Intensity_IW3_VV_slv1 still empty, the RGB only presents Intensity_IW3_VV_mst1

ABraun · January 2, 2019, 2:14pm

I can’t reproduce this error, sorry.

xydf_1992 · January 3, 2019, 1:14am

I just solved the problem, I changed the Back-Geocoding DEM, I used my local DEM for Back-Geocoding. I think maybe my error is caused by the Great Firewall of China. Thank you very much!

xydf_1992 · January 3, 2019, 5:32am

The Unwrapping result is as fellow:

After Phase to Elevation:

It’s black again.

ABraun · January 3, 2019, 7:35am

Phase to elevation also uses a DEM.
Change it to SRTM 3Sec or ACE30, for example

xydf_1992 · January 3, 2019, 8:33am

I finally made a DEM .

But the elevation ranges from 4000 to -4900 , that doesn’t match reality.

ABraun · January 3, 2019, 10:32am

The extreme values don’t matter. They result from unwrapping errors and atmospheric disturbances of the phase signal. Have a look at your coherence layer. It tells you where the quality of the DEM is trustful.
If there is vegetation in your area, it is unlikely that you will receive a perfect DEM from repeat-pass InSAR because of temporal decorrelation.

xydf_1992 · January 4, 2019, 3:17am

Could I do atmospheric correction before S1 TOPS Coregistration？ How can I see the quality of the DEM whether it’s trustful or not? Could you give me more details? Thank you very much.

ABraun · January 4, 2019, 8:31am

SNAP doesn’t support atmospheric correction.
Noise cannot be removed, only coarsely approximated subtracted from the phase. This is only possible with large stacks of images, for example by estimating the atmospheric phase screen (APS, short explanation here). This technique is not really suitable for common interferometry, but rather for persistent scatterer techniquest to derive surface deformations.

Values of high coherence tell you where your data is suitable. Please have a look at this post: Some explanations about concepts of fringes of interferogram and coherence

Besides that, ESA provides nice explanations on interferometry: http://www.esa.int/About_Us/ESA_Publications/InSAR_Principles_Guidelines_for_SAR_Interferometry_Processing_and_Interpretation_br_ESA_TM-19

Here you will find three documents:

Part A is for readers with a good knowledge of optical and microwave remote sensing, to acquaint them with interferometric SAR image processing and interpretation.
Part B provides a practical approach and the technical background for beginners with InSAR processing.
Part C contains a more mathematical approach, for a deeper understanding of the interferometric process. It includes themes such as super resolution and ERS/Envisat interferometry.

xydf_1992 · January 7, 2019, 5:34am

S1A_IW_SLC__1SDV_20171217T100443_20171217T100510_019741_02190D_231D.zip
S1A_IW_SLC__1SDV_20171229T100443_20171229T100510_019916_021E78_13C8.zip
these two made the DEM201712
S1A_IW_SLC__1SDV_20180709T100447_20180709T100514_022716_02762B_5BB2.zip
S1A_IW_SLC__1SDV_20180721T100448_20180721T100515_022891_027B8F_7120.zip
these two made the DEM201807
They both use the same reference DEM
Why the result value ranges gap so large?
What’s the DEM elevation unit？ meter?

ABraun · January 7, 2019, 7:55am

The unit is meter, yes. It is adjusted to the DEM used in the phase to elevation step.

There are many reasons why InSAR results are mostly not usable. The most important are temporal decorrelation, atmospheric effects and unwrapping errors. Here is a nice compilation of what can go wrong: https://eo-college.org/resources/insar_errors/

ABraun · January 17, 2019, 12:53pm

an example how atmospheric effects are reduced in time-series data of Sentinel-1 is given here:

source and full text: https://www.int-arch-photogramm-remote-sens-spatial-inf-sci.net/XLII-2-W7/597/2017/isprs-archives-XLII-2-W7-597-2017.pdf