Phase to Displacement Units?

hai @dsmilo , according to your suggestion about zero point on project area i want to ask, if my interferogram is want to know the displacement of mount eruption, then what point i have to choose? bcs my project area is tropical country (covered by vegetation), instead my radar source is sentinel-1 which carried band C. thanks before!

Hi. I have a quastion, when i calculate vertical disp as you mentioned in your shot, after applying geometric terrain convertion, my disp map appear black and white!! What can go wrong during band math calculations, i dont know!!pls help me

Hi @zealandia_sarah, I am having a little trouble understanding your question. I think you are asking where to find your zero point in such a high vegetative site. In my opinion if you can’t find an area that has no change to help you create a benchmark for all your data, the amount of error will go up in your results. High vegetation can create a whole lot of decorrelation which can skew your results pixel by pixel. It would be hard to call your results reliable. Do a little more geographic research in your AOI and see if you can find a feasable zero benchmark. You can also use other data to help. Try to see if you can find survey data, well data, contour data from similar time periods to help orient your dataset. Are you trying to track the change in the ground under the canopy or is there a bare earth or near bare earth space that you are trying to detect change at?

You always want to apply the geometric terrain correction last. You want to apply vertical displacement calculation after you import you unwrapped image back into SNAP. Order of operations is really important here. Remember that when you orient your image using geometric terrain correction, you are changing the units from radians to meters. Your image is born in radians and all work done to your image in SNAP needs to stay in radians until you are done. If you terrain correct your image that changes your units to meters, then you apply a tool that works in radians, you will have some messed up results. Hope this makes more sense.

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@zahra0729: Did you have a look at the histogram or the image statistics if the values are still ther? Maybe you just need to refine the colour manipulation.
Edit - solved here: Phase Unwrapping to Vertical Displacement

hai @dsmilo i’m sorry my question didnt complete. what i want to know is how to find a zero point for vert disp reference if my AOI is full of vegetation. if i use survey data from GPS (elevation) is that reliable? thankyou for your answer

vegetation is very bad for interferometry. Search for ares with high coherence to find suitable points.

It will be very hard to do that in an AOI full of vegetation. In my opinion, whatever results you do get probably won’t be very good to be honest. Remeber the concepts of how satellites collect data for interferometry. It all goes back to that. If the data isn’t collected well and correctly, then you are starting out with a bad dataset to begin with. Vegetation creates interference during data collection.

Hi, i have a quastion, i want to analayse ground deformation through a year for a region, so i have created displacment maps by generating interferogeram of all 12 month of year such as slave image and image of the first month such as master image,I mean in whole during of analysing,my master image was the first month image and the other images was slave.but now i,m confused if have i done it right or not!is it possible that I use from two images in different season!!!

Hi, I want to analayse ground deformation through a year for a region, so i have created displacment maps by generating interferogeram of all 12 month of year such as slave image and image of the first month such as master image,I mean in whole during of analysing,my master image was the first month image and the other images was slave.but now i,m confused if have i done it right or not!is it possible that I use from two images in different season!!!

so far, i made 2 vertical displacement maps from SENTINEL-1 nov 2014- feb 2015 and feb 2015-april 2015. i want to make band vertical map refference as @dsmilo told about. this is my band vertical displacement processed with band math:

as you can see, the minimal value in blue (-924 cm) and the highest value in dark red- black (32 cm). when i tried to find point of refference, because my area of interest is in tropic area, which covered by vegetation, therefore i choose the land use (resindent area) for point of refference. so, the vertical displacement overlay with the land use shp in ArcGIS. what i want to ask is which point i have to choose, according to picture, resident area (yellow) is spreading, some of them in higher value, some of them in smaller value. thankyou

We cannot tell you which point is suitable als a reference, this is based on field experience and knowledge on the local conditions.
Again, look at the coherence, as I suggested above. The reference area should be a point with high coherence.


Hi @ABraun , I would like to ask. According to @zealandia_sarah’s post above, the settlements spread around the volcano (at the center of image). We assumed the settlements have high coherence. If the reference took place at the south-west (which vertical displacement is lower than the volcano’s), the volcano would be an uplift area, and if the reference took place at the north-east (the higher value), it would be vice versa. Which one the reference point that we should choose? Thank you

to me it seems that there is a ramp in the unwrapped interferogram that doesn’t reflect the actual conditions. But you guys are the experts for this area - to me it is just colors… I don’t even have an idea about scale.
So, I would advise not to blindly trust these colours and values and think if the results are reasonable at all. I am no expert in volcanism, you will have to check if it is likely that one side of the volcano is uplifted while the other subsides. As these patterns are quite large, I rather think there is an atmospheric distortion or unwarpping error superimposing the actual changes.
Don’t just assume high coherence at cities, check it.

@zealandia_sarah Hi there, Just by looking at your displacement image results, I can tell that the unwrapping didn’t work properly probably due to low coherence. It is not normal for a result to hold very low values in one corner, and increase to a high number in an even fashion all the way to the adjacent corner. This looks like error to me. Land use linework will not help you with this. The purpose of this image is to show you how the land had changed between 2 time periods. The very first thing you want to do is check the modeled coherence of the coregistered pair of images before you run the interferogram tool. If this number is pretty far from 100% coherence, there is no point in moving forward with that pair of images. This can be a very frustrating step in interferometry, but it’s your fist sign that will direct you to move forward or not.

To find your zero, you can use google earth to find mountains or areas that haven’t changed. @ABraun is right, finding a zero area is not easy in an area with high vegetation. You need to have good understanding of your project area and understand it’s geology to find bedrock areas.

On another note, I see that you are using data from 2014 and 2015. I would be very cautious using data close to the satellite launch date of April 2014. I have found that the data is not very good until late 2015/early 2016. With any new technology, it takes time to work out the bugs and errors. Just be extra cautious. Checking coherence is a must.

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Hi Johngan,

thank you for sharing. I am currently converting my unwrapped phase to LOS displacement and also vertical displacement. To my understanding the vertical disp after conversion should be smaller than the LOS disp itself (due to the pythagoras relationship). The formula should probably look like this
vert_disp = (unwrapped phase * wavelenght*cos(rad(incidence angle)))/ (-4pi )

or I can be wrong, can you comment on this please?

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Below, i tried to sketch the relationship between LOS and vertical displacement (not the ideal diagram though). In order to find the real displacement, we need to do some geometrical calculations between the incidence angle and the direction of the ground movement. In the (beautiful :stuck_out_tongue: ) diagram below, the blue line denotes the LOS displacement and the red line the vertical displacement (this is what we are trying to find.).

using your formula vert_disp = (unwrapped phase * wavelenght*cos(rad(incidence angle)))/ (-4pi ) the results we get are much smaller compared to the following formula (unwrapped phase * wavelength) / (-4 * PI * rad(cos(incidence angle))). If the ground in your area of interest has moved vertically(we must assume that the horizontal component is zero) , then the vertical displacement values should be bigger compared to that of LOS.

You can have a look at the following website ( where after ALOS measurements, they found out that the amount of subsidence is 1.3 times the amount of deformation in LOS direction.

In order to validate this equation, the only way is to have precise GPS measurements over your area of interest and compare the GPS results with your vertical displacement map. As long as scientists use the following equation (unwrapped phase * wavelength) / (-4 * PI * rad(cos(incidence angle))), then that means that the equation was validated and produces the expected results.



Thanks for your post it was a great help. However, in reviewing the process in preparation for new images I noticed the formula you used (and I used successfully as well) is slightly different than the one often quoted in these forums in various threads.
Instead of: (unw_phase * wavelength) / (-4 * PI * cos(rad(incident_angle)))
Others have stated it is (unw_phase * wavelength) / (-4 * PI * rad(cos(incident_angle)))

These of course give different results that are differently signed.
(The second version, at least in my AOI, gives wildly unrealistic movement results and in the opposite direction of ground referenced data.)

Any insight you, @ABraun or others might have in this would be helpful.

Thank you,

It depends on whether cos() ingests and outputs degrees or radians. The units need to be in radians when you multiply with 4*PI.

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Makes sense of course. Thanks @mengdahl

It seems then, and I guess it would be good to find clarification on this, that SNAP’s band math cos() expects radians as input. (Much like excel). This would suggest the rad(cos version referenced on these forums was being used outside of SNAP’s band maths expression editor.

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