SNAPHU Unwrapping


Hello osvaldoguax,
Congrats for the DEM extraction work.
I wish to have Sarscape s/w too. Can you share it or is there any open source version of it.


Hello guys,

I carried out phase unwrapping of my interferogram (left) and produced an output (middle) that looks strange and incorrect, compared to the 1-sec DEM from SRTM (right). Is there anywhere I did wrong?

and my output in terminal looks like this

Please help! much appreciated.



Hello Glenn,

Congratulations for getting the final output image.

Can u walk me through the process after “SNAPHU Import” because I’m getting an error "Error: Java Null point exception " while doing the SNaphu import process.


Hi all!

After working a few months with interferometric data (PALSAR and Sentinel-1) my advice is to wait until ESA starts to delivery Sentinel-1B data. The interferometry between Sentinel-1A SLC scnes, as well as other SAR missions, depends on two basic aspects: 1. correct perpendicular baseline (there is a minimum and a maximum, that should be respected according to different SAR Systems). Values below or above those limits would result in poor quality DEM; 2. The lost of correlation due to temporal baseline is significant and should be considered. It is better to use SLC pairs with a short temporal baseline. That’s why I think It would be better to run the interferometry process with SLC pairs of scenes acquired over the same region by Setninel 1A and Sentinel 1B. I believe these pairs would have a more appropriate temporal and perpendicular baselines, since they were planned for this propose.


Hi shubham,

thanks. Did you select the correct files for snaphu import? The wrapped phase image should be the one you run the snaphu export with (.dim), while the unwrapped phase image should be the output of your phase unwrapping (.hdr). Assuming that your phase unwrapping runs correctly, you should be able to produce your final output.



Hi Glenn,
Thanks for your response.
Yes, I selected the correct files for the SNaphu Import,But still I’m stuck with the same problem.


Hi shubham,

I have no idea then. Maybe @iveci can help?


I choose *.hdr file for unwarped input and it works ok


Thanks a lot for your guidance @glenn @agruno and valuable information. It wouldn’t have been possible without your guidance.
It worked and i’m getting a nice Geocoded interferogram image :slight_smile:


Unrelated question, but is your wrapped phase in the proper geometric projection? If so, how did you do it–I’m trying to do it for an Sentinel 1 interferogram but having zero success (geometrically corrects everything but the wrapped phase image which disappears from the band list).

Thanks, MEH


Hi Mark.
yes, it is geometrically correct. I used the function of Range Doppler Terrain correction. And don’t forget to select(tick) the Output complex data in your processing parameters. You shall see your wrapped phase in the band list.


and also you can select output band of DEM, which you can compared against your interferogram.


Hi osvaldoguarx,

May I know the exact steps of how you average the resulting DEMs? Many thanks



For those of us learning interferometry still, what would be a good range for the perpendicular baseline of Sentinel 1 images?
Thanks, MEH


I have read in the reference for the Sentinel-1 (the 3 parts of ERS tutorial) that the recommended perpendicular baseline for interferometric images is between 150 and 300 meters. Low perpendicular baselines make easy to unwrap but they have a lot of decorrelation due to termal and atmospheric noise, bigger values of baselines is more or less the opposite.


Hello Glenn,

did you understand why your unwrapped phase looks this way?
I have the exact same problem and i dont know what is going on …



Hi osvaldoguax,
I am desperately trying to generate my own Sentinel-1 derived DEM, and am also having limited success. I see later down this forum you successfully achieved a better result using SarScape software. However in this post you managed this result with SNAP. I have a couple of questions I would be grateful if you could help with:

  • Which polarisation did you use? (does it even matter)?
  • Did you use the 1 second arc SRTM during back geo-coding? Or the default 3sec arc? Or even your own externel DEM?
  • Did you add any additional processed to the standard graph process on SNAP graph-builder? I added goldstein filtering with default settings.
  • With regards to the image pairs, I have found that the 12 day repeat observation did not overlap entirely (slight offset by about half a burst). The 24 day repeat however overlaps perfectly. Did you use 24-day separated pairs or a mixture?

Sorry, turns out I had a few more than just a couple of questions. As you can see, a lot of uncertainties.



Hi all,

I am trying to use snaphu to unwrap my data. The following appears after running

ziad@WIN764-716040R /cygdrive/c/vm/vmshare/targetsnaphu/interferogram-formation
$ snaphu -f snaphu.conf Phase_ifg_srd_12Apr2009_14May2009.snaphu.img 5001;

snaphu v1.4.2
27 parameters input from file snaphu.conf (84 lines total)
Logging run-time parameters to file snaphu.log
Creating temporary directory snaphu_tiles_6692
Unwrapping tile at row 0, column 0 (pid 5548)
Unwrapping tile at row 0, column 1 (pid 4936)
Unwrapping tile at row 0, column 2 (pid 992)
Unwrapping tile at row 0, column 3 (pid 4496)
Unwrapping tile at row 0, column 4 (pid 3424)
Unwrapping tile at row 0, column 5 (pid 3632)
Unwrapping tile at row 0, column 6 (pid 5292)
Unwrapping tile at row 0, column 7 (pid 1468)
Unwrapping tile at row 0, column 8 (pid 3752)
Unwrapping tile at row 0, column 9 (pid 3864)
Unwrapping tile at row 1, column 0 (pid 5180)
Unwrapping tile at row 1, column 1 (pid 6600)
Unwrapping tile at row 1, column 2 (pid 2612)
Unwrapping tile at row 1, column 3 (pid 6320)
Unwrapping tile at row 1, column 4 (pid 4492)
Unwrapping tile at row 1, column 5 (pid 5288)
Unexpected or abnormal exit of child process 2612



Hi There

I face the same problem…

Unexpected or abnormal exit of child process

any help?

Export Mask Pixels with snappy

Increase row and column tiles numbers in snaphu export. Default is 10 put 20. Works for me.


Hello everyone,

I have been working with SNAP and images SENTINEL TOPS since February of this year, first with images of my city (Bogota, Colombia), for coherence problems, switch to the Guajira Peninsula (northern Colombia) almost deserted zone, but I didn’t have good results with unwrapping images.
Finally, I solocite to Scott Henderson (team mate) an area that has been working with ISCE to compare the results with SNAP and the result of the DEM obtained is very different from the SRTM.

Joining to requests from slybacon (Alisdair), Saraka (Sahra) and Glenn, could we obtain the basic procedure used by osvaldoguax, for the DEM shown on 15 November last year?
A question for mfoumelis: Can we get an updated tutorial to produce DEM in SNAP?