QGIS Raster calculator - if statement

Question

I'm fairly new to QGIS, but I'm getting the hang of it. I have two layers - one DEM and one raster made from shape file (Buildings), but need one with higher Elevation where the buildings are for terraflow. But I can't seem to get the raster calculator to work with if Statement. I found a Workaround by assigning a fixed elevation value to the building raster, that is higher than any Point on DEM.

But on DEMs with high Elevation difference this means that some Buildings are over 50m tall!!! would like to make an if statement like this:

A B

A = DEM

B = Buildings

if(B>A,A+10,A) 

Answer 1

QGIS raster calculator uses a different logic for if statements. For implementing this expression if(B>A,A+10,A), you need to write in Raster Calculator Expression Window following formula:

( "B@1" >  "A@1" )  *  ( "A@1" + 10 ) + ( "B@1" <= "A@1" ) * "A@1"

and select output layer as C.tif.

I tested it with two arbitrary random raster and it worked as expected. It was corroborated by using Value Tool QGIS plugin.

Answer 2

With the release of QGIS version 3.22 Białowieża these days, the long awaited, simpler conditional statement is introduced, using the new if function: see changelog and documentation.

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As noted in the commnents below: The QGIS native raster calculator (the one under discussion here) can be found in Menu Raster > Raster Calculator

It can be found, alternatively, in the processing toolbox (Menu Processing > Toolbox), where apart from this one, there are two other raster calculators: GDAL and SAGA. SAGA raster calculator has an ifelse condition included for a long time already, see e.g.: https://gis.stackexchange.com/a/383687/88814.

Answer 3

This is a great achievement (adding the IF statement). Just wanted to add a helpful tip as it is a little confusing where to find this tool. There are 3 versions of the Raster Calculator in 3.22 QGIS. 2 (earlier versions) can be found in the Processing toolbox but do not contain the new IF functionality. The third one (that contains the new functionality) is found under the Raster menu and not in the Processing toolbox. Perhaps in a later version some rationalisation could be done. However, well done to all involved in adding a much needed functionality.