Making interferogram for TANDEM-X bistatic mode

Dear @ABraun

I chose my area (figure1) and I downloaded them and based on the link you sent me, I should put them in below folder (figure2);

.snap\AuxData\DEMs\ASTER 1sec GDEM



I put unzipped folder there although that big unzipped folder has several zipped folders (figure3) but when I chose again ‘ASTER’ for converting unwrapped result to elevation, it did not work.

My guess is, maybe I should change something in ‘ASTER 1sec GDEM’ folder and then try converting unwrapped result to elevation.

Note: my goal is using ‘ASTER 1sec GDEM’ but I do not know something I’ve downloaded is exactly that one or not;

I’ve downloaded ‘ASTER Global Digital Elevation Model V002’

they alll have 0 KB, I think the download failed.

After you downloaded all required tiles, you can open them in a GIS, make a mosaic and use it as “External DEM” instead of selecting ASTER in the dropdown.

There was something wrong with zip folder of all data that earth data sent to me, now, I used the link and downloaded all and put in folder (figure 1). I think i should do something until ASTER dem can be active because I repeated processing and again it was failed.


what happens when you right-click > Add Elevation Bands and now select ASTER?

I did it (elevation-2 band) but there is nothing there. Maybe I should unzip folders in ‘ASTER 1sec GDEM’ folder?

I haven’t tried it myself but according to the documentation, you downloaded the correct data and put them into the right folder.

A similar issue was reported (and somehow solved) here: Iceland Aster DEM Coregistration Auxdata

How about my solution on manually creating a mosaic from the DEM tiles and use this as an external DEM?

OK. I read what he did here but anyway he did not explain

How can creat a manually mosaic from DEM?

One more thing about ACE30 DEM,

I did not work with DEM result before, but in my idea, when I subtract the downloaded DEM (ACE30 DEM) _figure1_from the result (my created elevation) _figure2_and create a difference image in the band maths. If the DEM has high quality, the difference should be very little.

Am I right? If so, based on difference result between DEM and my elevation result, the difference is between (-3.32 to 18.35)figure3.

I think it is too much! Is’not it?



Or maybe, I should do some calibration first on my elevation result (for instance, subtract height value at a stable point like airport here and add 13 m (resulting in sea level reference))) and then compare my elevation result with ‘ACE30 DEM’?

load all ASTER files into SNAP and combine them into one large raster with the moasic tool.

Comparing your InSAR elevations to the ones from the DEMs is not a good idea because they wer created based on different data and methods. Additionally, if they have different spatial resolutions, they are not suitable for subtraction from your InSAR elevations.

But you need a DEM for proper terrain correction of your InSAR elevations and I would suggest to use ASTER.

There are some ways to do mosaicing in SNAP but as our data are not SAR products, I think we can only use mosaicing in SNAP.
I used below steps;
Raster…Geometric operation…mosaicing

Then I added all zip files and I chose UTM zone_parameter projection=4_ here (figure 1) and when I went to another step (variable and condition), then I got this error(figure2).



Is my way right? Shall I use something for variable and conditions? How about ‘map projection definition’?

Firstly, you will have to stick to “Geographic Lat/Lon WGS84” as an reference system, don’t use UTM CRS (explanation is here: Georefrence in snap)

The other thing is, you have to select a variable. In your case, it is just “band 1” as each Tif has only one band:

All of this is described in the help section, do you even look at it? :slight_smile:

Yes I read it but my problem is that when I press on variable folder (the picture in figure1) then it does not open a band chooser.


have you entered all tiles in I/O Parameters?

Yes but my format is zip in ‘source product’.

then try extracting them and add the rasters instead.

Ok…l managed to make it by your help but lets explain my step to be sure, everything is OK;

In ‘map projection definition’, I left stuff like below (figure1);

Then my result is saved in ‘BEAM-DMAP’ format (figure2):

then I converted unwrapping file to elevation(figure3);

Figure 3.

And the result is here (figure4);

Is it OK?

1 Like

very good!
I want to be mentioned in the publication then :wink:

Exactly, you helped me a alot. It is my pleasure:slightly_smiling_face:.

Some more notes:

  1. How can compare my elevation result with ‘ASTER DEM’. There is a option to adding band for ‘ASTER DEM’ but it is empty. Shall I subtracted the mosaic I made from my elevation result for checking quality?
  2. You told me:

As a last step, you can apply Tange Dopper Terrain Correction so both elevations are projected into a coordinate reference system Then you can stack them and compare the elevations.

Shall I do it now?
I tried to do it (figure 1) but them my result is like this (figure2). I think something is missing in image.



Another thing that is weird for me is here;
Why when i applied terrain correction by using external file, the result is bigger that my elevation result. Please look at below;