Difference between vertical and LOS displacement

sure, I will post the results and make a comparison between the results derived using local incidence angle and the results derived using the incidence angle over the ellipsoid.

Hi again @mengdahl,

I just want to correct myself. For deriving the vertical subsidence, I have used indeed the incidence angle from ellipsoid (and not the local incident angle as I first mentioned. Sorry for the confusion) as shown below which varies from 32.4 to 34.4 deg. While the local incidence angle varies from 19 to 42 deg.

I guess it would not make much sense to use the local incidence angle for deriving the vertical displacement, as it varies a lot as you mentioned.

@johngan Using the formula for calculating the displacement in LOS you provided in your first post I obtained strange results. Using unwrapped phase in the range -37 to 18 the calculated displacement is quite different compared to the displacement obtained with the SNAP function . Any idea?

Thank you.

Hi Hriston+bg,

I imagine you are referring to the following equation: (phase * wavelength) / (-4 * PI)
When you use SNAP tool for deriving LOS displacement, the units are in meters. Hence, if you want to get exactly the same numbers in LOS with that of SNAP, when using the equation make sure that the units in wavelength are in meters. If you enter the units in cm, the results will be in cm.

Using either the equation or the SNAP toolbox the results should be the same

@johngan Could you please put the number you set? I used wavelength=5.6 for cm as units.

please, take a look at this post. Phase to Displacement Units?

The values for the equation are the following:
(unwrapped phase * 0.056) / (-4 * PI).
the results will give you LOS displacement in meters just like SNAP

@johngan Thank you!! I’ll try it.

This article is pertinent to the discussion at hand:

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Hi all,

Sorry but, I am confused just the issue of wavelenght, How can you see the value of wavelenght or can you calculate this value?

Best regards

Wavelength depends on the sensor, you can find it with Google. You can also look up the radar frequency from the abstracted metadata and compute wavelength = speed of light/frequency.

Hi @mengdahl,

I am using Sentinel 1B IW SLC data. Does anyone know the wavelenght of this data?


The S-1 center frequency is 5405MHz (see abstracted metadata), you can easily calculate the wavelength from this as explained above :slight_smile:

I obtained 0.055 meter but i want to see it in the article or conference paper to give reference.

Here’s a reference for the 5.405GHz Centre frequency:

Sorry for rising up this post, but I checked with both formulas above. The displacement is fine, but with the vertical one calculated with formula provided (unwrapped phase * wavelength) / (-4 * PI * rad(cos(incidence angle)) )the result is not as expected (see below). The incidence angle is taken from the tie-point grid band incident_angle.

Any suggestions?

24h latter - resolved using the average of the LIA from incident_angle band .

In SNAP after clicking menu item ‘radar>interferometric>unwrapping>snaphuImport’ there will be a box with 4 tabs ‘read’, ‘read unwrapped phase’, snaphuImport’ and ‘write’. But in GraphBuilder’s window after adding the same item as an icon there are only 2 tabs, ‘read’ and ‘snaphuImport’. I can add a ‘write’ tab but can not find the item corresponding to ‘read unwrapped phase’ tab. Is it available in SNAP?

the solution with the graph builder is obsolete. If was from a time when the import wasn’t as far developed.
You can just use “Snaphu Import”

hi mengdahl, i am new here,sorry to interrupt, i want to know where i can find the local incidence angle? thanks before

if your data was processed correctly, you find it under tie-point-grids (can also be used in the band maths):

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hi Abraun, we meet again, thanks! fyi my LOS DinSAR show in meter units, how if i want to change in cm units?