Conditional operation with two raster files

Question

I need to perform an operation on a raster file. My first attempt was with mapcalc in GRASS, but I got lost with the way it handles the 0 values, I am around 0, there migth be many zero values. I am working with double numbers. Maybe, it can be done in QGIS, which will be nice.

In simple words, this is like adding a building on a surface, but limiting it to 2 m and modifying anything if there no building or it is below surface.

The input:

• Raster1: a regular dtm with some null values in the bounduaries (I could have it in many format)
• Raster2: a same size and resolution but with a small surface within non null values of the previous raster

The logical operation, raster3 is the result:

if raster1 = null then raster3:=null
else 
  if raster2 = null then raster3:= raster1
  else 
     if raster2 <> null then
       if raster2-raster1>0 and raster2-raster1<2 then
          raster3:=raster2
       else
          if raster2-raster1>=2 then raster3:=raster1+2
          else raster3:= raster1            

It looks that I have a code, but I have never coded anything in GRASS or QGIS

Answer 1

As I do not have appropriate dataset, I have not tested this ... hopefully you get the idea.

("raster1@1" != -9999) * (("raster2@1" = -9999) * ("raster1@1") + ("raster2@1" != -9999) * (((("raster2@1" - "raster1@1") > 0) * (("raster2@1" - "raster1@1") < 2)) * ("raster2@1") + ((("raster2@1" - "raster1@1") >= 2) * ("raster2@1" + 2.0)) + ((("raster2@1" - "raster1@1") <= 0) * ("raster1@1")))) + ("raster1@1" = -9999) * ("raster1@1")

To test, please copy and paste the above expression into your QGIS Raster Calculator.

("raster1@1" != -9999) * (                   // if raster1 is not null then
    ("raster2@1" = -9999) * ("raster1@1")    // if raster2 is null then take raster1
    +                                        // or
    ("raster2@1" != -9999) * (               // if raster2 is not null then 
        (
        (("raster2@1" - "raster1@1") > 0) * (("raster2@1" - "raster1@1") < 2)   // if delta is gt 0 and lt 2
        )
        * 
        ("raster2@1")                        // then take raster2
        +                                    // else
        (
        ("raster2@1" - "raster1@1" >= 2)     // if delta is equal or gt 2
        *
        ("raster2@1" + 2.0)                  // then take raster2 + 2.0
        )
        +                                    // else
        (
        ("raster2@1" - "raster1@1" <= 0)     // if delta is equal or lt 0
        *
        ("raster1@1")                        // then take raster1
        )
    )
)
+
("raster1@1" = -9999) * ("raster1@1")        // if raster1 is null then return null (i.e. raster1)

As this is assuming your nodata cell is represented by -9999, please modify the number as necessary.