When I save and reopen a raster in terra, I am losing the assigned units. Perhaps there are some filetypes that can save them while others can't, but I haven't found documentation of that.
library(terra)
A <- rast( nrows=10, ncols=10, xmin=0, xmax=10 )
values(A) <- 1:100
units(A) <- "Percent"
writeRaster( x=A, filename="test.tif" )
B <- rast( "test.tif" )
B
units(B) # empty
As far as I know only the netCDF standard supports writing units to files (among raster file formats). The GeoTIFF standard and the data model of the GDAL library used for reading and writing raster data do not have the concept of units as far as I can see.
I have added a work-around for this problem in terra version 1.5.0 (currently the development version available from github)
Both units and timestamps are now saved to file, and with your example I get:
library(terra)
#terra version 1.5.0
A <- rast( nrows=10, ncols=10, xmin=0, xmax=10 )
values(A) <- 1:100
units(A) <- "%"
writeRaster(A, "test.tif", overwrite=TRUE)
B <- rast( "test.tif" )
B
#class : SpatRaster
#dimensions : 10, 10, 1 (nrow, ncol, nlyr)
#resolution : 1, 18 (x, y)
#extent : 0, 10, -90, 90 (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 (EPSG:4326)
#source : test.tif
#name : lyr.1
#min value : 1
#max value : 100
#unit : %
units(B)
#[1] "%"