crs.RdGet or set the coordinate reference system (CRS), also referred to as a "projection", of a SpatRaster or SpatVector.
Setting a new CRS does not change the data itself, it just changes the label. So you should only set the CRS of a dataset (if it does not come with one) to what it *is*, not to what you would *like it to be*. See project to *transform* an object from one CRS to another.
# S4 method for SpatRaster
crs(x, proj=FALSE, describe=FALSE, parse=FALSE)
# S4 method for SpatVector
crs(x, proj=FALSE, describe=FALSE, parse=FALSE)
# S4 method for character
crs(x, proj=FALSE, describe=FALSE, parse=FALSE)
# S4 method for SpatRaster
crs(x, warn=FALSE)<-value
# S4 method for SpatVector
crs(x, warn=FALSE)<-valueSpatRaster or SpatVector
logical. If TRUE the crs is returned in PROJ-string notation
logical. If TRUE the name, EPSG code, and the name and extent of the area of use are returned if known
logical. If TRUE, a message is printed when the object already has a non-empty crs
character string describing a coordinate reference system. This can be in a WKT format, as a <authority:number> code such as "EPSG:4326", or a PROJ-string format such as "+proj=utm +zone=12" (see Note)
logical. If TRUE, wkt parts are parsed into a vector (each line becomes an element)
Projections are handled by the PROJ/GDAL libraries. Recent changes in the PROJ library to improve transformations between datums have degraded the library's usability. The PROJ developers suggest to no longer use the proj-string notation to define a CRS, but use the WKT2 or <authority>:<code> notation instead. These alternative systems work for formally described CRSs that are in databases, but they do not cover the infinite number of CRSs that exist. It is not practical to define one's own custom CRS with WKT2. Moreover, unlike the proj-notation, these newer systems are hard to read and that leads to code that cannot be easily understood and, therefore, is more error-prone.
It is still possible to use the PROJ-string notation with one major caveat: the datum should be WGS84 (or the equivalent NAD83) -- if you want to transform your data to a coordinate reference system with a different datum. Thus as long as you use WGS84, or an ellipsoid instead of a datum, you can safely use PROJ-strings to represent your CRS; including to define your own custom CRS.
You can also set the crs to "local" to get an informal coordinate system on an arbitrary Euclidean (Cartesian) plane with units in meter.
character or modified SpatRaster/Vector
r <- rast()
crs(r)
#> [1] "GEOGCRS[\"WGS 84\",\n DATUM[\"World Geodetic System 1984\",\n ELLIPSOID[\"WGS 84\",6378137,298.257223563,\n LENGTHUNIT[\"metre\",1]],\n ID[\"EPSG\",6326]],\n PRIMEM[\"Greenwich\",0,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8901]],\n CS[ellipsoidal,2],\n AXIS[\"longitude\",east,\n ORDER[1],\n ANGLEUNIT[\"degree\",0.0174532925199433,\n ID[\"EPSG\",9122]]],\n AXIS[\"latitude\",north,\n ORDER[2],\n ANGLEUNIT[\"degree\",0.0174532925199433,\n ID[\"EPSG\",9122]]]]"
crs(r, describe=TRUE, proj=TRUE)
#> name authority code area extent proj
#> 1 WGS 84 <NA> <NA> <NA> NA, NA, NA, NA +proj=longlat +datum=WGS84 +no_defs
crs(r) <- "+proj=lcc +lat_1=48 +lat_2=33 +lon_0=-100 +ellps=WGS84"
crs(r)
#> [1] "PROJCRS[\"unknown\",\n BASEGEOGCRS[\"unknown\",\n DATUM[\"Unknown based on WGS84 ellipsoid\",\n ELLIPSOID[\"WGS 84\",6378137,298.257223563,\n LENGTHUNIT[\"metre\",1],\n ID[\"EPSG\",7030]]],\n PRIMEM[\"Greenwich\",0,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8901]]],\n CONVERSION[\"unknown\",\n METHOD[\"Lambert Conic Conformal (2SP)\",\n ID[\"EPSG\",9802]],\n PARAMETER[\"Latitude of false origin\",0,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8821]],\n PARAMETER[\"Longitude of false origin\",-100,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8822]],\n PARAMETER[\"Latitude of 1st standard parallel\",48,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8823]],\n PARAMETER[\"Latitude of 2nd standard parallel\",33,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8824]],\n PARAMETER[\"Easting at false origin\",0,\n LENGTHUNIT[\"metre\",1],\n ID[\"EPSG\",8826]],\n PARAMETER[\"Northing at false origin\",0,\n LENGTHUNIT[\"metre\",1],\n ID[\"EPSG\",8827]]],\n CS[Cartesian,2],\n AXIS[\"(E)\",east,\n ORDER[1],\n LENGTHUNIT[\"metre\",1,\n ID[\"EPSG\",9001]]],\n AXIS[\"(N)\",north,\n ORDER[2],\n LENGTHUNIT[\"metre\",1,\n ID[\"EPSG\",9001]]]]"
# You can use epsg codes
crs(r) <- "epsg:25831"
crs(r, describe=TRUE)$area
#> [1] "Europe between 0°E and 6°E: Andorra; Belgium - onshore and offshore; Denmark - offshore; Germany - offshore; Jan Mayen - offshore; Norway including Svalbard - onshore and offshore; Spain - onshore and offshore"
crs("epsg:25831", describe=TRUE)
#> name authority code
#> 1 ETRS89 / UTM zone 31N EPSG 25831
#> area
#> 1 Europe between 0°E and 6°E: Andorra; Belgium - onshore and offshore; Denmark - offshore; Germany - offshore; Jan Mayen - offshore; Norway including Svalbard - onshore and offshore; Spain - onshore and offshore
#> extent
#> 1 0.00, 6.01, 82.45, 37.00