DEM generation

[Ailin]

Alright, will try that and let you know then, thanks

[Ailin]

Hi ABraun, this is how it looks like:

[ABraun]

is this the case after the debursting?
Did you then have a look at the unwrapped phase?
Did you chose TOPO for the SNAPHU export?

[Ailin]

Hi!
@ABraun yes, that is after debursting. I chose TOPO and MCF for export.
The unwrapped phase (right after unwrapping) looks like a nice painting :

@falahfakhri I tried to do all the process without subsetting but run out of memory twice (8GB RAM is not much). Is there a problem in subsetting the image?

[Igor]

Hi Ailin,

You can’t generate a normal DEM, when you have low values of Bperp (slide 13 from https://earth.esa.int/documents/507513/0/InSAR_Tutorial_ERS-Etna/41625d92-93d0-41e4-8c7e-c96450d82fdb – 66.39 m). In fact, the document InSAR_Tutorial_ERS-Etna presents only the order of the steps! But the scene is like there (1 day in example) to get a DEM is not used for the case Sentinel-1! Bperp should be as large as possible. Usually to build a DEM choose scenes obtained with a low Btemp and perhaps a large Bperp.

For example, on page 132 ( http://www.smu.edu/-/media/Site/Dedman/Academics/Departments/EarthSciences/PDF/Lu/098_Lu_etal_InSAR_DEM_2012.ashx?la=en ):
“Assuming we have more than about 20 repeat-pass SAR images, we can generate a stack of N coregistered multitemporal interferograms. Because our goal is to generate a new DEM or update an existing low-resolution or low-accuracy DEM, interferograms with short time separations and large baselines are preferred. This is critical to derive a high-accuracy DEM.”

So, to correctly build a DEM, you should have a lot of different scenes. And already from it it is necessary to choose a pair that best meets the criterion of building DEM.

Best regards,
Igor.

[ABraun]

“Assuming we have more than about 20 repeat-pass SAR images, we can generate a stack of N coregistered multitemporal interferograms. Because our goal is to generate a new DEM or update an existing low-resolution or low-accuracy DEM, interferograms with short time separations and large baselines are preferred. This is critical to derive a high-accuracy DEM.”

So, to correctly build a DEM, you should have a lot of different scenes. And already from it it is necessary to choose a pair that best meets the criterion of building DEM.

thanks for the input, Igor. Multi-temporal data can surely increase the quality if interferometric analyses.

I was trying to do exactly what you describe but I am no longer sure if this is possible in SNAP:

Interferogram averaging for DEM generation

I wondered if it's possible (and reasonable) to calculate multiple interferograms and average them for later phase to elevation calculation. I have Sentinel-1 A/B interferograms (6 days temporal baseline each) in an arid region with high coherence. I would like to exclude atmospheric effect by a combination of interferograms. But I struggle to do it in SNAP. When would be the best step to do the averaging? Directly on the filtered interferograms (I doubt that this makes sense) After unwrap…

Based on the publication provided by mengdahl here, interferograms should be combined after unwrapping:

Interferogram averaging for DEM generation

A quick search found this:

Does anyone have an idea how they could be co-registered in SNAP?

[aminevsaziz]

@Ailin you can try 2 higher order consecutive Goldstein filtering but with lower values typically in logarithmic scale to ensure smoothing the results as much as possible while keeping fine details but with 8 GB of RAM & large scene it would be kind of difficult also I’m not so sure if this work in stbx1 but give it a try …

[aminevsaziz]

@ABraun I’m digging a similar subject this days about averaging stacks …I would say it’s possible on paper to implement it on snap as separate enhancement but it’s not easy as it sounds.

[falahfakhri]

the main idea of subset is to reduce the scene size, in case of S1-A/B, the better is to split and choose only the bursts of your study area, still the question is raised about the coherence of your study area, take a look at this

The quality of your dem is strongly affected by coherence and atmospheric disturbances.

Take a look at this

Problem in Snaphu Import section

For DEM-generation high coherence is needed, say 0.75 and above. Even with perfect coherence the DEM will still suffer from distortion caused by atmospheric artefacts. These artefacts can be mitigated by averaging many generated DEMs.

[Ailin]

Thanks to all for your replies.

You can’t generate a normal DEM, when you have low values of Bperp

You are right Igor, I read that the optimum Bperp should be between 150 and 300 m (mine is 24m)
So I think it is not possible to build a DEM with Sentinel images. Also, the area of my work is mountainous and with low coherence values in some parts. And SNAP does not allow to work with different subswaths (eg: IW1 and IW3) so I can not combine Sentinel 1 and 2 images…

@aminevsaziz I used consecutive Goldstein filters with a subsetted part of the image but had the same results as the image I posted.

in case of S1-A/B, the better is to split and choose only the bursts of your study area,

Yes, I used one burst.

So, should I try doing interferograms averaging, or is it worthless?

[falahfakhri]

Would you please to try out the whole image if that possible following the same processing chains, and let see what is the results? In this case there is no need to subset

[Ailin]

Yes Falah, I will, although I tried and run out of memory but will try again
Let you know then, thanks

[panda1]

Hello,do you solve this problem?

[mangla1991]

Hi,
I have done phase unwrapping successfully in order to create dem generation. The phase to height conversion is not working. There is another option called phase to elevation conversion.
Which process will i prefer for DEM generation?

and I am using TANDEM-X interferometry pair data. Is anybody has made DEM in SNAP?

[5h0hr3h]

hi, as i understand, you should use “phase to elevation” operator.

[falahfakhri]

Please take a look at this topic answers your question.

Phase to elevation vs phase to height

phase to height converts the unwrraped phase to the heights in the radar coded system so you need ground points with known altitude to converts your results into real ground values. phase to elevation uses a digital elevation model in reference height calculation. This operator converts interferometric phase to digital elevation map. Both operators are correct.

[5h0hr3h]

Hi @ABraun
i made a DEM in SNAP and convert it to GeoTIFF format . i intend to use this file in ArcMap. based on fig below, how to remove black borders (No data value) from this DEM?

Thanks you

[ABraun]

you can define a NoData value in ArcMap directly in the symbology tab. Check the box and all 0 values become transparent.

[5h0hr3h]

Thanks for quick responding. i did this earlier, but it didn’t work for me. i also tried with the procedure that is mentioned in fallowing website: http://support.esri.com/en/technical-article/000010059
but it didn’t work too.

[ABraun]

What value is the black area in your raster?