And dielectric constant (moisture)… That’s the real issue. We are running some test. NDVI,RVI and EMSI. There is probably no correlation, however we are using Sentinel 1 images and formula created for RADARSAT images, so one would that formula need some modification. It also seems to us that it very much depends on ascending or descending position of satellite.
Hi Tom, can you please share your research?
In general, we can use like {10^((Value in db)/10)} to convert the backscatter from dB to linear units. May I know the reason to use absolute (ABS) in the conversion?
Would you please to share the source of this equation, many thanks.
siva_iirs is referring to tomcater’s post at #7 in this topic: Creating Radar Vegetation Index
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For dual-pol Sentinel-1 like data, RVI can be formed by 4*VH/(VV+VH). Here, VV and VH are in linear power scale, not in dB.
Be careful with the processing step (old SNAP7.0 version produces opposite channel c11==VV and c22==VH). However, latest version is working fine.
Read: Covariance Matrix C2 for Sentinel-1 --Diagonal Elements
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Do you mean linear power scale can be obtained from covariance matrix C2 for Sentinel-1?
Yes, they can be obtained from C2 matrix. However, in a direct approach, radiometric calibration also provides Sigma0_VV and VH.
I disagree. You can derive T or C matrix out of the initial scattering matrix S, but not vice versa.
@ABraun Hi,ABraun.
Could you explain what the white area with high value( > 1) means in rvi image?
Rvi band
VH band
VV band
And I tryed servel sentinel-1 GRD scenes for rvi. Whatever there is white strip white area like above,there are always many pixeles with value greater than 1.
Does it make sense ?
Thanks for your time!
It looks like Radio Frequency Interference (RFI). There is a transmitter on the ground that is generating C-band RF emissions that are disturbing S-1 image formation.
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Thanks for your reply. Do you have any idea about the pixels with value greater than 1 exept RFI?
At the moment,I don’t know how to handle those pixels.
The RFDI was developed based on all four polarizations. If you adjust the equation because you are using Sentinel-1 with only VV and VH, it is not surprising that the value range is a bit different. Unlike the NDVI it is not strictly limited to -1 and +1 but rather a relative measure of the presence of vegetation. Also, it fails at artificial structures, such as urban bodies.
Use the mask manager to identify RFDI pixels > 1 and check where in the image they are.
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Sir,it seems that there is still no obvious threshhold between RFDI pixels and other normal pixels with value > 1.
mask by > 1
mask by > 1.3
And I read some discussion and know there are no way to remove RFDI currently.
So this makes things harder,have to make a median composite and interpolate by previous and post scenes.
you could mask out these areas entirely to exclude them from the median composite.
Hi,
I tried to use RVI using GEE platform, but find a problem to de convert the unit from dB to linier scale, does anyone know the right script to do it?
thank you
In the link bellow the RVI is calculated a bit differently (using S1 GRD products).
Do you think you can possibly explain the difference between the two formulas ?
What are they represent and what are their differences?
@ABraun Please help, can anyone tell me why my value range on RVI is from 0-255 and not from 0-1? How do I get the range to be similar to NDVI? I’ve noticed that (everyone) else managed to get the range from 0-1 (approximately).
I have followed all of the pre-processing steps explained in this thread.
I used the 4VH/(VV+VH) formula in band math
Thanks!
Here’s a version translated using Yandex. The formatting isn’t great, but it’s understandable. I wonder if this formula can be applied to ALOS HH/HV data instead of VV/VH. I’ll give it a try.
16-24(1).pdf (337.2 KB)