Let's say I have a SpatRaster data containing 2 variables. One of them is called "elev", and the other is called "relative_elev":
library(terra)
f = system.file("ex/elev.tif", package="terra")
r1 = rast(f)
r2 = rast(f)
varnames(r2) = "relative_elev"
r_combined = c(r1, r2)
Checking the variable names:
varnames(r_combined)
#[1] "elev" "relative_elev"
I want to subset data based on their names. When I do r_combined["elev"] I expect to get only the elev data, but I get both "elev" and "relative_elev":
r_combined["elev"]
#class : SpatRaster
#dimensions : 90, 95, 2 (nrow, ncol, nlyr)
#resolution : 0.008333333, 0.008333333 (x, y)
#extent : 5.741667, 6.533333, 49.44167, 50.19167 (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 (EPSG:4326)
#sources : elev.tif
# elev.tif
#varnames : elev
# relative_elev
#names : elevation, elevation
#min values : 141, 141
#max values : 547, 547
When I do r_combined["relative_elev"] I get an error:
r_combined["relative_elev"]
#Error: [subset] no (valid) layer selected
Both behaviours are very unexpected and confusing to me.
names refers to layers, varnames to variables or sub-datasets. varnames can be used to extract a sub-dataset (variable) from a SpatRasterDataset.
s <- sds(r1, r2)
s
#class : SpatRasterDataset
#subdatasets : 2
#dimensions : 90, 95 (nrow, ncol)
#nlyr : 1, 1
#resolution : 0.008333333, 0.008333333 (x, y)
#extent : 5.741667, 6.533333, 49.44167, 50.19167 (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 (EPSG:4326)
#source(s) : elev.tif
#names : elev, relative_elev
s["relative_elev"]
#class : SpatRaster
#dimensions : 90, 95, 1 (nrow, ncol, nlyr)
#resolution : 0.008333333, 0.008333333 (x, y)
#extent : 5.741667, 6.533333, 49.44167, 50.19167 (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 (EPSG:4326)
#source : elev.tif
#varname : relative_elev
#name : elevation
#min value : 141
#max value : 547
To be able to subset like this, perhaps you should read your file with sds(filename) instead of rast(filename).