# how to understand that the SNAP results are correct

Hello everyone,
I used SNAP in order to calculate the deformation of an earthquake. My final results show that the deformation is between -0.003 and 0.04 meters. There is a free website (http://sarviews-hazards.alaska.edu/), which calculates earthquakes’ deformation by GAMMA software. The deformation between -0.098 and 0.031 was calculated by the website for exactly the same InSAR data. Why is there such a difference between GAMMA results and SNAP results? and how can I understand that my results from SNAP are correct?

I don’t think it makes much sense to look at the extreme values, because they can strongly differ depending on

• the size of the investigated area (larger area means potentially higher outliers)
• the unwrapping method (some are prone to errors from low coherence areas while others have a smoothing effect)

Until you can not guarantee that these are also the same in your case, the differences are not very suprising. And even then, the unwrapping has a random comonent which can result in different outcomes for each run.

What matters if the median displacement between both calculations is more or less the same. And even then, the size of the inspected area has an impact when you compare the results. Do the patterns of displacement look the same?

Thank you for your response. Actually, the investigated area is the same for both processes, but the patterns of displacement are relatively identical.

then we can exclude the size as one reason for the difference.

But as GAMMA was used for SARVIEWS and you used SNAP there could be many parameters which potentially introduce differences, e.g. Orbit files, coregistration accuracy, filtering techniques, flat earth estimations, unwrapping, resampling…

Also important: Which reference height was used in both products? The relative deformation might be the same for both approaches, but the zero-level can be a different one (which would explain the different minima/maxima).

If the overall pattern is similar and makes sense (!) I see no reason for question the SNAP output. But it is always good to bear in mind the many error sources of InSAR products: https://eo-college.org/resource/insar_errors/

How did you retrieve the min/max range of deformation in SNAP?

I retrieved min/max of displacement_vv from color manipulation tab.

the Color Manipulation tab does not show the real min/max values, because it scales the data over 95% of the range to increase contrasts.

Please use the statistics tool to see the real minimum and maximum values.

Thank You for information

My 5 cents to this topic.
For the event I worked on (https://earthquake.usgs.gov/earthquakes/eventpage/us70006d0m/executive#executive) I also found some discrepancies, but I stayed with SNAP results. For EQs it is important to have the same data SAR
processed. I used ASF for generation of IFIs with GAMMA and the results are not the same. I also couldn’t get pixel size below 40x40m while SNAP gave me 15x15m.
As pointed @ABraun you have limited control over the results one gets from ASF or other online tools.