[GIS] Replacing 0 values of a raster with values from another raster in QGIS

Question

What I want to do is pretty straight forward: I have two raster with the same crs, resolution and extent. Both rasters show the number of days an area is flooded so they just have two values. Raster a consists of 0 and 183; raster b of 0 and 149. Logically, the area that is flooded only 149 days (b) is a little bigger than the area that is flooded 183 days (a).

Now I want to combine both maps. The formula I think should work using the saga raster calculator is:

ifelse(eq(a,0), b, a) .

In my opinion it translates to "If a pixel of map a is equal to 0 then use the pixel value of map b if not, keep the pixel value of a" So the result should be a map having just three values: 183, 149 and 0, right? But no, the resulting map has all kind of weird (i cannot explain them) values where map a is 0 and map b is 149. It seems like some transition from 183 over 149 to 0.

Why is that, or is it a bug?

Ps: Here is an example for one of the grey pixels of the calculated map: a=0, b=149 but the calculated map has a value of 128.30

Answer 1

Solution using SAGA Raster Calculator

I performed a quick test and you should probably use this expression:

ifelse(a=0,b,a)

instead of the one you provided (also, choose 8 byte floating point number as Output Data Type parameter).

Using these sample rasters:

and the provided formula, I get:

where gray zones are equal to 149 and white zones are equal to 183 (there are also few cells with different values in the upper left of the raster, but I think it is due to the way with which I reproduced the problem, so the starting rasters maybe were not perfectly aligned).

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EDIT

Solution using QGIS Raster Calculator

I have just found another approach, which seems to work. Use the QGIS Raster Calculator instead of the SAGA Raster Calculator: you will find it from Raster >> Raster Calculator.

Apply the first calculation using this expression:

("a@1"=0)* "b@1"

where a@1 is the raster A and b@1 is the raster B. This operation replaces all the 0 values from the raster A with the values from the raster B.

Then, do a second calculation on the previous result using this expression:

"result@1" + "a@1"

You will get this result:

as expected (I hope).