Using nested conditional statements in QGIS Raster Calculator

Question

I'm currently working on a project in QGIS and have encountered an issue with formulating a nested conditional statement in the Raster Calculator.

How do I resolve this?

My goal is to perform a raster calculation that prioritizes data from three different raster layers ("National_rp@1", "District@1", and "Water@1"), and uses slope data from a fourth raster ("Slope_rw_rp@1") only if all three of the other raster layers have null values for a particular cell.

I have tried:

if(isnull("National_rp@1") AND isnull("District@1") AND isnull("Water@1"), 
   6 * 2.71828 ^ (-3.5 * abs("Slope_rw_rp@1" + 0.05)), 
   if("National_rp@1" = 1, 90, 
      if("National_rp@1" = 2, 50, 
         if("District@1" = 1, 70, 
            if("District@1" = 2, 30, 
               if("Water@1" = 1, 9999999, 0)
            )
         )
      )
   )
)

But I get a generic Raster calculation error.

Answer 1

Short answer: convert NoData values to a numerical value, then use an equal (=) statement to get a binary (0/1) output that you can use to multiply the result instead of nester if-statements.

In detail: Working with NoData cells is tricky and the syntax isnull("District@1") is not a valid expression. QGIS raster calculator does not seem to have an operator to treat NoData cells (see also here) - I don't know from where you have the isnull() operator.

You can use Fill NoData cells to convert NoData pixels to a fixed value (e.g. 999999) and then use this one in the raster calculator to see if the raster value is equal to this, in which case you get 1: ("National_rp@1" = 9000) - when the output is 1, this means the raster value is NULL. Do this for all three rasters and multiply the results: if all three values are 1, you get 1. If one of the values is 0 (thus: one of the three raster is not NULL), the output is 0

    if (
        ("National_rp@1" = 9000) * ("District@1" = 9000) * ("Water@1" = 9000) = 0,
        etc.

Continue like this to get 0/1 results instead of nested if-conditions.