SNAPHU Unwrapping

thank you Mr ABraun

how can I to add two phases unwraping in snap?

how Can i add two interferograms?

we discussed this here: Interferogram averaging for DEM generation

Hi Abraum
when I finish the interferogram generation and the layer of displacement I have a rating of colors and with values. I want to know what is the correct form of interpretation the values pisitive and negative. are the values positives elevation of terrain, and values negativ subsidence?

I recommend reading this topic: Some explanations about concepts of fringes of interferogram and coherence

Hi, what is the boundary of error of the InSAR?or the margen of error?

That is quite a general question. The smallest measurable displacement depends on the wavelength of the sensor (have a look at “height of ambiguity“ in the ESA documents above).
Further aspects include spatial resolution, perpendicular baseline (“critical baseline“) and the accuracy of the orbit information.

So it’s hard to give a number here, but these are some great sources:

I do snaphu unwrapping and it generates the next product, but I do not think it’s right about the phase. Then the DEM that generates is not the correct one either, for the study area. Could someone tell me what is happening?

I’m afraid the quality of your inteferogram is not sufficient to generate a fully covered DEM. As you see, there are larger areas in your image with low coherence (no fringes). During the unwrapping process, thesse areas are not correctly converted to an absolute phase. What parameters (TOPO, DEFO, SMOOTH) did you use for unwrapping?
What is the temporal baseline of your image pair?
To get a proper DEM you need high coherence throughout the entire image.Can you please post an image of your coherence (including the color bar)? Additionally, the incidence angle and the perpendicular baseline (can you please check how large it is?) play a role.

Thank you for your reply.
The parameter that I use is DEFO.
Temporal baseline: 6 days (Master:30-03-2019 Slave:24-03-2019).
Track: 68.

As you see, the coherence is quite low - do you have much vegetation in your study area? The lower half is completely decorrelated.

You can use the InSAR stack tool to get information on the perpendicular baseline.

Although it will probably not change very much, you should use TOPO in the unwrapping settings for DEM generation grafik

Thank you. That low coherence may be due to the temporary basis or the type of soil ?. It’s a maritime coast, that’s why there is a lack of coherence in the lower part (I must remove it). Then there are mountains / elevations of sand, lagoons and fields.

hello, is it true that SNAP has released a plugins for unwrapping? so we didnt need the virtual machine to unwrap? Plus, i found a video on youtube on how to use the plugin. Link below:

I have follow the tutorial and it runs exactly like the video. I have been running it for 6 hours and didnt get any result yet. Can anyone confirm that this method is working or not? Thank you.

Yes, indeed, Please take a look at the following post,

Source of the post

But for long time running, Please check up the following things,

SNAP update,
The RAM of your Machine
The processor might also affects,
The memory saving of your Machine

Continuing the discussion from SNAPHU Unwrapping:


snaphu v1.4.2
usage: snaphu [options] infile linelength [options]
most common options:
-t use topography mode costs (default)
-d use deformation mode costs
-s use smooth-solution mode costs
-f read configuration parameters from file
-o write output to file
-a read amplitude data from file
-c read correlation data from file
-b perpendicular baseline (meters)
-i do initialization and exit
-l log runtime parameters to file
-v give verbose output
–mst use MST algorithm for initialization (default)
–mcf use MCF algorithm for initialization

type snaphu -h for a complete list of options

E:\SNAPHU\snaphu-v1.4.2_win64\bin>snaphu -f snaphu.conf Phase_ifg_VV_18Oct2014_01Jan2019.snaphu.img 17618

snaphu v1.4.2
27 parameters input from file snaphu.conf (84 lines total)
only one tile–disregarding multiprocessor option
Logging run-time parameters to file snaphu.log
Reading wrapped phase from file Phase_ifg_VV_18Oct2014_01Jan2019.snaphu.img
No weight file specified. Assuming uniform weights
Reading correlation data from file coh_IW2_VV_18Oct2014_01Jan2019.snaphu.img
Calculating deformation-mode cost parameters
Building range cost arrays
Building azimuth cost arrays
Initializing flows with MCF algorithm
Setting up data structures for cs2 MCF solver

it stucks at here for 1 day. im using i5 8400, 8gb ram running on windows 10. where did i get wrong?

The RAM 8 GB , is not enough, Also the processor might be causes a problem, but coming back to the RAM issue, there are two solutions, first one you could subset your AOI, second one is , increasing the SNAP RAM as explained below by @marpet
Or you could try up both solutions,

For the SNAP Desktop application, you can increase the amount of memory available to SNAP.

In the ‘etc’ folder of the SNAP installation directory, you’ll find a file named snap.conf. Open it in a text editor.

There is the line which starts with ‘default_options=’

In this line you’ll find an option like -J-Xmx5G . Increase the value. You could use something like -J-Xmx13G , if you have enough memory in your computer. By default, it is set to ~75% of the maximum value. This is usually a good choice.

Problem solved!! I just multilook the image before unwrapping. Does multilook cause the result to be different? Im just curious since my system cant run the non multilook image. So i cant tell the difference.

Multilook decreases the image size quite drastically (depending on your parameters) and also increases the phase quality (lower speckle). In general, it results in Multilook increasing the quality of the unwrapping process while decreasing a lot the processing time.

Except if very high fringe rate, I recommend always multilooking before unwrapping. With low amount of RAM (which is your case), this recommendation becomes a requirement.


as @qglaude answered your question

in here,

That’s mean, the source of your problem in your machine is the RAM, and the processor, So, subset, multilook, increasing the virtual RAM, should be taken in account in your future processing.