Flood detection using SAR

Hi all,

I’m having some doubts regarding flooding extraction from SAR images.

I am using two IW _GRDH products: one, on the left, is a pre-event image the other on the right is a co-event image.

Why does the pre-event image is so dark with such a huge quantity of pixels with low values of backscattering? I am supposing this image is not rappresenting any flooding event or maybe this consideration is wrong?

Two images have same relative orbit, both ascending. I’d use them for change detectiong too but I got back really weird behaviors.

Did I choose the wrong reference image?

what is the date of both images?

Looks like the contrast ist different for the display. Please set the same range of values for the greyscale in the Color Manipulation Tab

Use exactly the same minimum/maximum values for both images here.
Double-click on the numbers to enter specific values.

Many thanks for your reply.
I did what you said and visualization is not changed:
the histogram of the reference image dated September 3rd 2019

and the histogram of the coevent image dated September 21st 2019

Dark areas on reference image (dated September 3rd 2019) have pixels values < -14 db: they look like water… how is this possible?

Radar images are dark if the ground is very dry for example. Did you try looking at ratio-images?

Have you used Sentinel-1 products from the same orbit? Maybe it depends on difference in acqusition geometry?

I noticed at your second histogram (September 21st 2019) that Max value is quite different then September 3rd 2019. In my opinion 5 dB is huge difference.

yes of course the images has the same relative orbit both ascending.

I was exploring a sentinel-2 image: actually dark areas correspond to bare soil.

I was thinking: bare soil appear dark when it is very dry and with higher values of backscattering when it is wet. This is because of differences of dielettric costant, is it right?

Is this behavior affecting the output of log ratio for flooding identification? I mean , these area could be classified as false positive

Yes that is correct. Also if the soil is rather smooth the return will be darker.

BTW you are not very limited in your pre-event image selection so you could for example download 3-5 scenes and average them for a lower-speckle high-resolution reference scene.