So, it is normal to get somewhat larger vertical measurements (VD) than the LOS measurements. In my case (using the given equation), I have a maximum difference of about 1cm (VD > LOS). I thought before that LOS measurements will always be larger than VD measurements.
I am measuring a longitudinal surface displacement of a road. I have used the cursor to move around the area to get the pixel information as well as using the line drawing tool. I did it several times for a number of pairs (14). I have the results already. They looked okay actually. But my supervisor, asked me if instead of doing this several times, if I can use a written script (say in Python) to get the pixel information along the line of pixels (road line) I am interested in. For example, in QGIS, I can call Python to do some tasks for me as long as I have a script.
Since SAR sensors are side-looking, it is the vector sum of both the vertical and horizontal components along the line of sight (LOS) that can be measured, creating some ambiguity in separating the two elements.
A displacement in the vertical direction will induce a displacement SMALLER when projected in the LOS direction. However a LOS displacement cannot guaranty that the displacement in indeed vertical.
I can take a example. In glaciology, when studying ice-sheet displacement, the ice-flow mostly follows the topography, which is very flat in some regions. You can observe by DInSAR a displacement. however, you know that this displacement in the LOS direction is due to horizontal movement instead of vertical.
I think the blue region shows the upper and lower limits (computed by sigma method?) and the dark blue line is the mean value. My band is named: vert_displacement_VV. Then I do get (not my own) the last two columns with _mean and _sigma.
When I move the cursor to the pixels of my interest, and compare the values from those extracted using the line drawing tool (in .txt), there’s no difference.
I see. Thank you for this information. I will revisit each pair I have.
I also would like to ask the following:
Say I have an area that doesn’t move vertically up nor down, meaning zero surface displacement at any time instant as supported by field measurements. Is it possible that when I do DInSAR, I will get non-zero displacements?
This depends on many factors,
What kind of area is it? ,
What kind of objects cover this area? ,
Is the processing 100% right?
Are the atmospheric’s effect excluded 100%?,
Dose the topographic phase is still related to the Perpendicular Baseline (PB) ?
Is the unwrapped step done correctly?
I am covering a road line, about 280m long, that is about 600m away from my main AOI, which is a road also suspected to have longitudinal surface displacement due to a tunneling activity. These 2 roads are parallel with one another.
For the processing, I think I did it correctly.
S1 TOPS Coregistration with ESD (includes S1 TOPS-Split and Apply-Orbit-File)
Goldstein Phase Filtering
Phase Unwrapping (SNAPHU Export and Import)
Phase to Displacement
Range-Doppler Terrain Correction
Extraction of pixel information using the line drawing tool to get Profile Plot
I am not sure about atmospheric effect exclusion, and relationship between topophase and PB.
We believe the tunneling activity started in early/late December but we got displacements prior to December. That is why I have considered a road far from my main AOI to check the results prior to the month of December. At first, I expected all displacements will converge but unfortunately, they are not after plotting the results.
Sorry if I confused you. And my apologies for saying that DInSAR may not give reliable results.
Below is the results I obtained for the road line (600m away from the main AOI) which we suspect that is not affected by the tunneling activity. This is related to my previous post. Our assumption is, since this road line it is not affected by the tunneling activity, it should not have any displacement (say DInSAR should give zero values). But I wonder if this is absolutely true when doing DInSAR. That even if we have absolute no ground movements, DInSAR might still give non-zero measurements. I know there are other factors to be considered here as you have enumerated awhile ago.