# Conditionals in the QGIS raster calculator

I needed to do some conditionals in the QGIS raster calculator, but it doesn’t support that—or at least doesn’t seem to. But it does support logical operators, with a result of either 0 or 1. For instance, here’s the script I wrote:

# Subtract them ((DavisQuad2012-02-25T16_00_00Z@1 - DavisQuad2012-02-29T16_00_00Z@1)* # Multiply by 1 if neither is 255 (NoData), 0 otherwise (DavisQuad2012-02-25T16_00_00Z@1 != 255 AND DavisQuad2012-02-29T16_00_00Z@1 != 255)) # Subtract 32768 if either one was NoData, giving us -32768 for NoData. - (32768*(DavisQuad2012-02-25T16_00_00Z@1 = 255 OR DavisQuad2012-02-29T16_00_00Z@1 = 255))

Of course, you can’t actually put the comments in. But what it does is this: First, I subtract one raster from the other and multiply that by the logical operation that neither one contains NoData. That gives me the difference of the rasters, or 0 if either one contains NoData. Then I subtract 32768 multiplied by the inverse of the aforementioned logical operation, so any pixel with a NoData value in either of the original rasters is -32768 in the new one.

Note: I expanded on an idea from the GDAL gdal_calc.py script.

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