min and max values displayed are different from those actually stored in a terra raster

Question

When and why do terra rasters sometimes display min and max values different from those actually stored in the raster?

 # create a 10x10 binary raster with values 0 or 10:
 r <- rast(nrows=10, ncols=10, vals=ifelse(runif(10*10)>0.5,0,10))
     
 # export it as a binary compressed file:
 r2 <- writeRaster(r, filename="r_CCITTFAX4.tif", overwrite=TRUE, gdal="NBITS=1",gdal = "COMPRESS=CCITTFAX4", datatype="INT1U")
 
 # values are known as 0 and 10, as expected:    
 minmax(r2)

 # > lyr.1
 # > min     0
 # > max    10

 # and:
 r2
 
 # > class       : SpatRaster 
 # > dimensions  : 10, 10, 1  (nrow, ncol, nlyr)
 # > resolution  : 36, 18  (x, y)
 # > extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
 # > coord. ref. : lon/lat WGS 84 (EPSG:4326) 
 # > source      : test_CCITTFAX4_INT1Ud.tif 
 # > color table : 1 
 # > name        : lyr.1 
 # > min value   :     0 
 # > max value   :    10 

 # but 'true' values stored in the raster are 0 and 1:
 table(values(r2))
 
 # >  0  1 
 # > 51 49 

Is this normal behavior or a misreading of this type of compressed geotiff file? Is it related to the color table that appeared between r and r2?

I know that I can force the values to be in agreement, using:

 setMinMax(r2, force=TRUE)

But that's not what I want. I'd rather get back the original 0 and 10 values in the raster r2. How can I do this correctly during or after reading the file?

Thanks for your help.

Jean-Luc

Answer 1

You use GDAL option NBITS=1

r2 <- writeRaster(r, filename="r_CCITTFAX4.tif", overwrite=TRUE, gdal=c("NBITS=1", "COMPRESS=CCITTFAX4"), datatype="INT1U")

That changes values of 10 to 1 when writing the file. When computing the min and max values, 10 is considered to be valid value in the INT1U range. There is no check if the values are changed on writing because of GDAL options like the one you used.

In this case you would have to transform the output raster first. With this example you could do

rr <- r != 0
rr2 <- writeRaster(rr, filename="r_CCITTFAX4.tif", overwrite=TRUE, gdal=c("NBITS=1", "COMPRESS=CCITTFAX4"), datatype="INT1U")
minmax(rr2)
#    lyr.1
#min     0
#max     1

Answer 2

Thanks Robert, that's clear. So, it's a kind of GDAL liberality: GDAL does check the consistency of the raster values with the chosen datatype parameter, but not with the nbits parameter.

So, if you want to recover the initial value (and as long as it wasn't outside the values allowed by INT1U), you can add, after reading the file:

r2[r2!=0] <- minmax(r2)[2]

It's a bit dangerous, but it gets the job done.