How to check the NAflag value in `terra`

Question

I need to know how NA values are coded in tif files because I am working with a program that cannot use tifs that have NAs coded as 0.

However, when using the NAflag() function in terra, I always get NaN as the output, even if I set NAflag() myself

Here's an example:

# Create a raster
test <- rast(x=matrix(c(1,1,1,1,1,0,0,0,0), nrow=3))

plot(test) # plotting shows 1s and 0s

# No NAflag set yet, returns NaN
NAflag(test)

# Set NAflag to 0
NAflag(test) <- 0

plot(test) # plotting shows 0s are now NA

# NAflag still returns NaN
NAflag(test)

Maybe I am misunderstanding how NAflag works. Is there another way to learn what value is being used as the NA value in a tif?

Answer 1

You can find that with terra::describe (a.k.a. GDALinfo)

library(terra)
f <- system.file("ex/elev.tif", package="terra")
v <- grep("NoData Value", describe(f), value=TRUE)
v
#[1] "  NoData Value=-32768"

strsplit(v, "=")[[1]][2] |> as.numeric()
#[1] -32768

NAflag is used to set this value prior to reading values from a file. For a SpatRaster with cell values in memory, it is always NaN.

r <- rast(f)
NAflag(r) <- 255
NAflag(r) 
#[1] 255

You are correct that it does not read the original NAflag (NoData Value).