is some data below the sea level or generally near the coast?
Some DEMs only feature land areas.
If you are insecure about the quality, you can subtract the downloaded DEM from the result and create a difference image in the band maths.
What do you mean by data? There is some ridges near coast that I try to detect.
I mean that parts of your study area might lay within areas which are not contained in a DEM because they are off the coast, which could be an explanation for why the ASTER GEDM failed.
There is a small part of land in my images and most of my area is ice. Can this be a reason? If yes, then why it worked with
‘‘ACE 30 (Auto download)’?
as I said - some DEMs are 0 over water areas (or ice) and some are NoData.
No data - no reference height - no result 
1 Like
another thing that came to my mind: You downloaded the ASTER data before applying this module, right?
As stated in the drop down menu, it is not Auto Download. That means, it must be downloaded manually and placed in the correct folder, as described here: External DEMs
Dear ABRUAN
No I did not download them.
I went to ‘earthdata’ site but there are somany zip folders. Is it important, which one I download?
you have define your area of interest in the map and the results are reduced to the tiles contained by this area.
OK. Thanks. I will download it but for now, I used ‘ACE30’ and my result for 29May in HH band is like below:
Figure1. interferogram from 29 May in HH band
Figure2. Unwrapped result from 29 May in HH band
Figure3. Elevation result from 29 May in HH band
This is my first experience about DEM with TanDEM-X data and I do not know how good is the result! but I do not know why there is a gradiant (i mean a color pattern from left to right _ yellow to blue_) on my elevation result?
looks very good. The could be caused by large along-track baseline of the pursuit mode. Maybe you find some indications on DEM quality from this mode in this publication:
Add the elevation band to your product as I explained above to compare the unwrapped result with the real elevations from ACE30. Definitively worth trying ASTER as well as it is of clearly better quality.
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
Figure1.
Figure2.
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.
Figure3.
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.
figure1
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?
figure1.
figure2.
figure3
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).
figure1
figure2
Is my way right? Shall I use something for variable and conditions? How about ‘map projection definition’?