Conditional calculation using raster calculator in QGIS

Question

I have two rasters with the same extent, resolution, and projection, let's say raster A & B. What I want to do is something like this:

IF A = 3 OR A = 11 THEN A+B
ELSE B

How can I do such a thing with a raster calculator?

Kazuhito · Accepted Answer · 2019-12-16 08:34:16Z

9

Try this syntax:

(A@1 = 3 + A@1 = 11)* A@1 + B@1

@1 means the first band of the raster
A@1 = 3 returns 1 (TRUE) if A is 3
When A is 3 or 11, (A@1 = 3 + A@1 = 11) part returns 1. Otherwise it returns 0.

answered Dec 16, 2019 at 8:34

Kazuhito

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I test it and it works just fine, but I don't really understand the A@1 + B@1 part of the syntax how can it returns B when the condition is false? Can you elaborate on that part?
– greensquare68
Commented Dec 16, 2019 at 9:39
@MuhamadNasirLukman When the condition is false (i.e. A is neither 3 or 11), both of the A@1 = 3 and A@1 = 11 return 0. Then the overall expression becomes 0 * A + B and ends with B.
– Kazuhito
Commented Dec 16, 2019 at 10:00

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