Difference between vertical and LOS displacement

[Duoc]

This is mt_prep_snap

[mdelgado]

You should not only look at the StaMPS log. Try to load any variable such as pscands.ij or similar and search 0 or NaNs inside.

[Duoc]

Dear @mdelgado,
I have checked pscands.1.ịj, pscands.1.da, pscands.1.ij0, but they do not have 0 or NaNs value and “pscands.1.ij0” does not value, does right?

[mdelgado]

Well, I am running out of ideas then.
Had you visually checked the interferograms to verify that all look ok?

Let us know

[gomalhunzai]

I have used 6 interferograms (between June 2016 and June 2017) where I stacked them and produced the subsidence map.

@ABraun @falahfakhri @johngan can you please sort out how this is done using SNAP?
Is it possible to use 20 pairs (A+B), (B+C) , (C+D) … so on which will produce separate stack for each pair and also separate interfergram. So my query is about how we can produce stack of all these interferograms at this stage for performing a time-series analysis?

[falahfakhri]

Please take a look at this topic,

Interferogram averaging for DEM generation

Calculate interferograms for images pairs of different dates (all at 6 day intervals at best) and stack/coregister them. You can then calculate the phase average in the band maths. However, I am not sure how the snaphu export will recognize this band maths result as the correct image to export and process within snaphu, because usually the input for the snaphu export has i, q, intensity, phase (and sometimes coherence). So we would need to find a work around to replace it by the averaged interf…

[gomalhunzai]

Thanks @falahfakhri, i read all the conversation btw u and andy.
At the end of the day we dont have any perfect methodology on which we rely.
What if we calculate the subsidence using No.of pairs (A+B, B+C, C+D, …) till last step (displacement) separately and averaged the results of all displacement layers.

[ABraun]

At the end of the day we dont have any perfect methodology on which we rely.
What if we calculate the subsidence using No.of pairs (A+B, B+C, C+D, …) till last step (displacement) separately and averaged the results of all displacement layers.

Currently, this is the best option available. You can also plot the displacements of the single dates (pairs) of a pixel along a graph.
An example is given at the end of these slides: https://eo-college.org/resources/insar_deformation/

[gomalhunzai]

An example is given at the end of these slides: https://eo-college.org/resources/insar_deformation/

i am affraid there is no pdf available for deformation

[ABraun]

you have to register (free) to download the materials.
The download button will then appear at the bottom

[gomalhunzai]

@ABraun can you please tell me why we have to add the reference point value and vertical displacemnt, in INSAR tutorial by Dr.-Ing. Diana Walter.
Also, does the sign of the reference value is important or not???

[falahfakhri]

I’m sorry to get a cross this discussion, but would you please to share the title of this tutorial,

[gomalhunzai]

@falahfakhri please follow below link to download the tutorial after signing in to the website.
https://eo-college.org/resources/insar_deformation/

[ABraun]

@ABraun can you please tell me why we have to add the reference point value and vertical displacemnt, in INSAR tutorial by Dr.-Ing. Diana Walter.

we have discussed this here:

• Subsidence map in 3d view
• Question regarding Phase to Displacement and Time series tool in SNAP

[gomalhunzai]

Thanks @ABraun i already read these topics. One thing which is confusing me is the “+” sign in the tutorial slides. As per you discussion in various topics regarding the Vertical displacement you have mentioned to subtract the value of pixel where no deformation happened.

Question regarding Phase to Displacement and Time series tool in SNAP

the reference height would be at a point where you have high coherence and expect no change in the image. The displacement value at this point is the one to to level to zero (by adding or subtracting the respective value). If you then mask out all areas with low coherence the total displacements along the LOS should remain .

[johngan]

HI @gomalhunzai,

In the tutorial you went through, the value that has been selected as a reference point is -8.821452 (negative number). The subtraction is as follows: unwrapped_phase - (-8.821452) which becomes +. Lets substitute the unwrapped_phase with a subsidence value, 8mm - (-8.821452mm) = 16mm.
Instead, what we can do is 8mm + 8.821452mm = 16mm

[gomalhunzai]

8mm + 8.821452mm = 16mm

@johngan then how we can say we have subtracted the values???
The value is doubled now.

[johngan]

The subsidence values you see on the final map are relative (master image relative to slave). In order to derive the absolute subsidence, we need to find an area in our relative subsidence map where we know that the subsidence is zero. Let’s say that you got GPS measurements for an area that you know no subsidence was occurred. You locate this area on your unwrapped interferogram and extract its value which happens to be -8.821452 (remember, this is relative).

Hence, we have the following: unwrapped_phase - (-8.821452) which becomes -8.821452 - (-8.821452) = 0

So, this is the area with a zero absolute displacement. By applying this value to the whole image, we can find the absolute subsidence

[gomalhunzai]

@johngan i am afraid that we subtract the relative or reference value from the image with vertical displacement. As shown in this picture.

[johngan]

The SAR sensor makes observation in Line of Sight (LOS) direction (this is the SAR geometry) as shown in figure below:

Fig 1: LOS direction

Hence, whatever measurements we do, the results are going to be in LOS direction. In order to derive subsidence in vertical direction (we assume that the subsidence occurred vertically and not east-west), we need to do some trigonometry. So, in the equation given in the tutorial you have read, the cos (theta) is added to the equation in order to get the results in vertical displacement and not in LOS. The displacement values though, are still relative (but in vertical direction not in LOS), which means we still need to subtract a value of zero dsiplacement to get absolute values.