R 'overlay' two rasters to create one single band raster and preserve cell values

Question

I have two single band rasters r1 and r2.

and

Put together in a GIS they look like this:

Raster extend and cell size of r1 and r2 match perfectly. The white areas are nodata cells. There is no overlapping of cells that have a cell value.

I would like to 'overlay' the two rasters to create one single band raster!

I already tried:

r_overlay <- overlay(r1, r2, fun=sum)

and

r_mean <- mean(r1, r2)

and

r_brick <- brick(r1, r2)

but none of the approaches seems to work. The goal is to create a single band raster containing the values of both rasters r1 and r2 and nodata values where there are nodata values now.

Answer 1

sum (or overlay with sum) works if you add na.rm=TRUE.

Test data - an upper and lower triangle with 5 or 7 and NA:

> r1 = 5*raster(upper.tri(matrix(1:64,8,8)))
> r1[r1==0]=NA
> r2 = 7*raster(!upper.tri(matrix(1:64,8,8)))
> r2[r2==0]=NA

Plot them - these should show triangles with a single value and empty cells:

> plot(r1)
> plot(r2)

Try adding them:

> plot(r1+r2)
> plot(sum(r1,r2))

And everything is NA because there's an NA in one of the two rasters everywhere. So exclude:

> rsum = sum(r1,r2,na.rm=TRUE)
> plot(rsum)

gives:

a complete square of 5s and 7s. If you had any parts with NA in both rasters, that would be NA in the output. If you had cells that were a 5 in one raster and a 7 in the other, you'd get a 12 in the output.