R language, unreasonable (?) raster resolutions - r

Question

Sorry for the very stupid question, but I'm really stuck here... I need to create a Digital Elevation Model for my study area. For this, I downloaded an SRTM (1 arc-seg resolution, freely available from the net) image, which comprises a region wider than my area of interest. The original raster has these characteristics:
class : RasterLayer
dimensions : 3601, 3601, 12967201 (nrow, ncol, ncell)
resolution : 0.0002777778, 0.0002777778 (x, y)
extent : -45.00014, -43.99986, -22.00014, -20.99986 (xmin, xmax, ymin, ymax)
crs : +proj=longlat +datum=WGS84 +no_defs
source : s22_w045_1arc_v3.tif
names : s22_w045_1arc_v3
values : -32768, 32767 (min, max)
I need to (1) increase the resolution (initially of 30.75662 * 28.68392 m) to 1 * 1 m (that is, I really do not care about the exactitude of the elevations) and (2) crop a squared area of 2000 * 2000 m centered at a given coordinate. So, the first step I'm following is to re-project to UTM:
projection(r) <- "+proj=utm +zone=23 +datum=WGS84"
But the resolution units do not change after that:
class : RasterLayer
dimensions : 3601, 3601, 12967201 (nrow, ncol, ncell)
resolution : 0.0002777778, 0.0002777778 (x, y)
extent : -45.00014, -43.99986, -22.00014, -20.99986 (xmin, xmax, ymin, ymax)
crs : +proj=utm +zone=23 +datum=WGS84 +units=m +no_defs
source : s22_w045_1arc_v3.tif
names : s22_w045_1arc_v3
values : -32768, 32767 (min, max)
If I try to set the resolutions in meters manually, then generates an empty raster. Can anybody be so kind as to throw some light on me here?

Answer 1

You are changing (i.e. overwriting) the CRS, not projecting the raster. Usually, it is recommended to create a template raster with the CRS and the resolution you need and reproject the raster using this template.
See here an example, I am switching to terra for the analysis since it is a newer and faster package, but I would show also how to convert it back to raster format:
library(raster)
#> Loading required package: sp
# Faking your data
r <- raster(
nrows = 3601, ncols = 3601,
ext = extent(c(-45.00014, -43.99986, -22.00014, -20.99986))
)
values(r) <- seq(-32768, 32767, length.out = ncell(r))
r
#> class : RasterLayer
#> dimensions : 3601, 3601, 12967201 (nrow, ncol, ncell)
#> resolution : 0.0002777784, 0.0002777784 (x, y)
#> extent : -45.00014, -43.99986, -22.00014, -20.99986 (xmin, xmax, ymin, ymax)
#> crs : +proj=longlat +datum=WGS84 +no_defs
#> source : memory
#> names : layer
#> values : -32768, 32767 (min, max)
plot(r)
# End of faking data
# Change to terra, much faster
library(terra)
#> terra 1.5.21
r_terra <- terra::rast(r)
template <- terra::project(r_terra, "+proj=utm +zone=23 +datum=WGS84")
# Change to the desired res
res(template) <- c(2000, 2000)
# Reproject
r_terra_reproj <- terra::project(r_terra, template)
r_terra_reproj
#> class : SpatRaster
#> dimensions : 56, 52, 1 (nrow, ncol, nlyr)
#> resolution : 2000, 2000 (x, y)
#> extent : 499985.4, 603985.4, -2433195, -2321195 (xmin, xmax, ymin, ymax)
#> coord. ref. : +proj=utm +zone=23 +datum=WGS84 +units=m +no_defs
#> source : memory
#> name : layer
#> min value : -32371.95
#> max value : 32182.81
terra::plot(r_terra_reproj)
# Back to RasterLayer
r_reproj <- raster(r_terra_reproj)
r_reproj
#> class : RasterLayer
#> dimensions : 56, 52, 2912 (nrow, ncol, ncell)
#> resolution : 2000, 2000 (x, y)
#> extent : 499985.4, 603985.4, -2433195, -2321195 (xmin, xmax, ymin, ymax)
#> crs : +proj=utm +zone=23 +datum=WGS84 +units=m +no_defs
#> source : memory
#> names : layer
#> values : -32371.95, 32182.81 (min, max)
Created on 2022-06-10 by the reprex package (v2.0.1)