How to open Landsat bit value quality flags rasters in R?

Question

Landsat QA rasters are stored in 8bit .tiff files. The flags are encoded by the bit position. But I can not extract them using terra.

I am trying to retrieve the bit position with this code:

library(terra)

toBinary <- function(i){
  valuesbin = paste0(as.integer(rev(intToBits(i)[1:8])), collapse = "")
  valuesbin = unlist(strsplit(valuesbin, split = ""))
  as.numeric(valuesbin)
  
}


qa = rast('./qarast.tiff')
qa = app(qa,toBinary)

but it crash with:

Error: [app] the number of values returned by 'fun' is not appropriate

How can I use this quality flags in R? Is there any specific library less error prone?

Cheers

Answer 1

toBinary is not vectorized

toBinary <- function(i){
  valuesbin = paste0(as.integer(rev(intToBits(i)[1:8])), collapse = "")
  valuesbin = unlist(strsplit(valuesbin, split = ""))
  as.numeric(valuesbin) 
}

toBinary(1:2)
#[1] 0 0 0 0 0 0 0 1

This is a vectorized function

toB <- function(x) {
    t(sapply(x, toBinary))
}
toB(1:2)
#    [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
#[1,]    0    0    0    0    0    0    0    1
#[2,]    0    0    0    0    0    0    1    0

And use it:

r <- rast(ncol=5, nrow=5, vals=c(NA, 1:24))
a <- app(r, toB)
#class       : SpatRaster 
#dimensions  : 5, 5, 8  (nrow, ncol, nlyr)
#resolution  : 72, 36  (x, y)
#extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 
#source(s)   : memory
#names       : lyr.1, lyr.2, lyr.3, lyr.4, lyr.5, lyr.6, ... 
#min values  :     0,     0,     0,     0,     0,     0, ... 
#max values  :     0,     0,     0,     1,     1,     1, ...