How to overlay two Rasters using ifelse (Conditional Statements) in R? - r

Question

I have two rasters (images), and want to overlay them using this code:
# Getting the images
library(raster)
URL1 <- "https://www.dropbox.com/s/6jjz7ou1skz88wr/raster_1.tif?dl=1"
URL2 <- "https://www.dropbox.com/s/d5xuixohjqfnfze/raster_2.tif?dl=1"
download.file(URL1, destfile=paste0(getwd(),"/", "raster_1.tif"), method="auto", mode="wb", timeout="6000")
download.file(URL2, destfile=paste0(getwd(),"/", "raster_2.tif"), method="auto", mode="wb", timeout="6000")
# Reading the images
raster_1 <- raster(list.files(pattern="raster_1.tif$"))
raster_2 <- raster(list.files(pattern="raster_2.tif$"))
# Overlaying
myFun <- function(x,y){ifelse(x==0 && y==0, 0, ifelse(x==1 && y==0, 2, ifelse(x==1 && y>0, y)))}
( res <- overlay(stack(raster_1 ,raster_2), fun = Vectorize(myFun) ) )
### R gives this error
Error in .overlayList(x, fun = fun, filename = filename, forcefun = forcefun, :
cannot use this formula, probably because it is not vectorized
I would be very grateful if anyone could help me.
Thanks.

Answer 1

You need a function that only uses vectorized operators. This is case where Boolean arithmetic should both succeed and be more efficient
myFun <- function(x,y){ 0*(x==0 && y==0)+
2*(x==1 && y==0)+
y*(x==1 && y>0) }
There are some edge cases that do not appear covered. Can x ever be a value other than exactly 0 or 1? Can y ever be negative?
After running my version I get:
> ( res <- overlay(stack(raster_1 ,raster_2), fun = Vectorize(myFun) ) )
class : RasterLayer
dimensions : 2958, 1642, 4857036 (nrow, ncol, ncell)
resolution : 500, 500 (x, y)
extent : -171063.8, 649936.2, 5317253, 6796253 (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=12 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
data source : in memory
names : layer
values : 0, 14751 (min, max)
I didn't think I would need to use Vectorize around myFun,enter code here but the results seems more likely to be correct when I leave it in the call to overlay:
> Hmisc::describe(values(res))
values(res)
n missing distinct Info Mean Gmd .05 .10 .25
3222508 1634528 1502 0.727 4918 6403 0 0 0
.50 .75 .90 .95
0 13898 14082 14168
Value 0 13000 13200 13400 13600 13800 14000 14200 14400
Frequency 2089448 67 578 10515 69031 249817 523241 226628 46191
Proportion 0.648 0.000 0.000 0.003 0.021 0.078 0.162 0.070 0.014
Value 14600 14800
Frequency 6876 116
Proportion 0.002 0.000
When I took out the Vectorize step I did not get an error but I got all zeros, instead.

Answer 2

It is not clear what you really are trying to achieve, and there might be better solutions. In your example data, Y (raster_2) has no values of zero. That suggest that you want the values of raster_2 where raster_1 is not 0? That can be achieved like this:
m <- mask(raster_2, raster_1, maskvalue=0)
I think that 42-'s myFun has a problem in that it returns 0 when none of the conditions are true, specifically when (x == 0 & y > 0)
To make it work with overlay, replace the && with &
myFunV <- function(x,y){
0*(x==0 & y==0)+
2*(x==1 & y==0)+
y*(x==1 & y>0) }
res <- overlay(raster_1, raster_2, fun = myFunV)
(but, again, I doubt that this is good approach for your needs)