Change raster cells non-NA values based on the longitude using R terra

Question

I would like to change all the non-NA cell values based on the cells longitude. For instance, any cell that had a non-NA value and a longitude greater than x would have the value replaced with y. I have found a way to do this based on the column index, but it is a fiddly and imprecise.

Here is an example of what I have managed to create based on the column index.

#Reading in a raster from the terra package 
f <- system.file("ex/elev.tif", package="terra")
r <- rast(f)
plot(r)

And I would like to change all the non-NA (i.e., all the cells that already have a colour in the above plot) that have a longitude greater than say 6, to have a new value of 500. Here is my solution based on column index:

r2 <- r2
#Here I have fiddled with the column index to arrive at a longitude of 6
r2[,32:95] <- 500
#Now I use the original raster to recode the cells in r2 which are no longer NA that were NA in r 
r3 <- terra::mask(r2, r)
plot(r3)

I suspect there is a more elegant solution but I can't figure it out. Any suggestions?

Answer 1

You could first create a longitude raster with init, mask that with your data and then use mask again, or ifel

library(terra)
r <- rast(system.file("ex/elev.tif", package="terra"))
lon <- init(r, "x") |> mask(r)

x <- ifel(lon > 6, 500, r)
# or 
y <- mask(r, lon>6, maskvalue=TRUE, updatevalue=500)

Answer 2

Well, not memory safe but, you do not need to create unnecessary objects.

Read data

library(terra)
r <- rast(system.file("ex/elev.tif", package="terra"))
  plot(r)

Create coordinate vectors that are ordered the same as the cells in the raster and then pass it to which to pull the cell indices associated with a query of the vector. The index xyFromCell(r, 1:ncell(r))[,1] is in reference to the first coordinate column (long) so, for lat it would be xyFromCell(r, 1:ncell(r))[,2]. The which function supports multiple criteria query using | or & and so, querying both coordinates can be done in one fell swoope.

r2 <- r #make copy 
  r2[which(xyFromCell(r, 1:ncell(r))[,1] > 6)] <- 500
    plot(mask(r2,r))