Read multiple layers raster from ncdf file using terra package

Question

I want to read a specific layer from a multilayer raster (format ncdf) using terra package. I want to extract each layer by polygon using terra package (it is a very fast way to extract values). I know there are other packages to open my multilayer raster but I don't want to use them. I want to use terra package but with multilayers I have many errors.

So, I tried:

 r <- terra::rast(r1,nlyrs=nlyr(4))## I have a error :unable to find an inherited method #for function ‘nlyr’ for signature ‘"numeric"’
 

I tried :

r <- terra::rast(r1, 1) ## no error   

but when I want to extract values from raster. I have this :

uat$ex <- terra::extract(r,vect(uat), fun = mean, na.rm =T)[,2]
 

###
 error: [extract] raster has no value

Answer 1

I don't see a way to read in only some layers from a NetCDF using terra::rast - it just doesn't take any extra args except subds when reading a file and that doesn't apply here, it seems.

The solution would be to read the NetCDF and then select the layers you need by name or number.

Read in:

> t1 = terra::rast("./s1.nc")

How many layers?

> dim(t1)
[1] 180 360   3

Names are?

> names(t1)
[1] "variable_z=1" "variable_z=2" "variable_z=3"

Get second layer by name:

> t1[names(t1)[2]]
class       : SpatRaster 
dimensions  : 180, 360, 1  (nrow, ncol, nlyr)
resolution  : 1, 1  (x, y)
extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +no_defs 
source      : s1.nc 
varname     : variable 
name        : variable_z=2 

Or equivalently t1[[2]] to get by number.

Note that the source here is given as s1.nc which I think means it hasn't read the NetCDF into memory yet, hence this is as efficient as reading a single layer would be in memory terms.

Answer 2

It works:

rss <- "/ncdffilename.nc"##  having 60 layers (it's a raster with one single variable for 60 years), the variable name is "cdd"
names(rss) <- "cdd" ## not necessary
r1 <- terra::rast(rss,subds = "cdd") ## select the variable name
r <- r1[[1]] ## give me the first layer