Use water mask to create subset

Sorry I am am quite new to Snap so please forgive my questions.
So I want to run a Chl_a Algorithm on some Landsat 8 images, and would like to use a water-mask to create a subset of just the water body so everything looks and works smoother.

Using fractional Land/Water mask created a new subset and now my question is how can i use this to cut out the water from the original subset? I tried to use the land/sea mask but i cant chose the new water mask as it is in a new subset. What is the easiest way to achieve my goal?
Thanks for your time

You question is absolutely valid. Nothing to forgive :slight_smile:

Currently it is not possible to use a mask for defining a subset region.
Probably you know your target area. So you can draw a rectangle around it. Then ensure the rectangle is selected and the right-click on it and select Geometry to WKT.
This will show you the WKT string. You can directly use it for example in the Subset operator.

isn’t the CR2CC algorithm automatically masking out land areas?

Edit: Obviously only large inland water bodies, such as Lake Victoria, for example.

That’s true. C2RCC computes only over water. But you still need to subset to your lake, for example.

ok thanks, so maybe to show what my problem is:

after cr2cc and running my chl_a algorithm, it computes the values also outside of the actual water body which is kind of iritating in othere examples where its not so easy to see the border between water and land. So my question was to only get the actual water. But i can work with this neverthless. thanks for your time!

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Ah. okay. For Landsat there is no good flag for discrimnating between land and water.
That’s why the validExpression parameter is empty by default.
You can make your own, by checking the radiance values for certain thresholds for example.
or you can merge the mask you have created into the landsat product and use this band in the expression.

okay thanks i will try that

just wanted to leave a short thank you for your constant help,

it worked perfectly with merging the water mask into the product and including it in the expression


Hi @flobert, I am currently trying to merge masks into a product as you seem to have done: can you detail the steps or the tool you used?