update CRS text in sf1; #2495

Author: edzer

vignettes/sf1.Rmd

@@ -578,38 +578,35 @@ functions are limited to PostGIS.
 
 ## Coordinate reference systems and transformations {#crs}
 
-Coordinate reference systems (CRS) are like measurement units for
-coordinates: they specify which location on Earth a particular
-coordinate pair refers to. We saw above that `sfc` objects
-(geometry list-columns) have two attributes to store a CRS: `epsg`
-and `proj4string`.  This implies that all geometries in a geometry
-list-column must have the same CRS. Both may be `NA`, e.g. in case
-the CRS is unknown, or when we work with local coordinate systems
-(e.g. inside a building, a body, or an abstract space).
-
-`proj4string` is a generic, string-based description of a CRS,
+Coordinate reference systems (CRS) are like measurement units
+for coordinates: they specify which location on Earth a particular
+coordinate pair refers to. We saw above that `sfc` objects (geometry
+list-columns) have a an attribute of class `crs` that stores the
+CRS. This implies that all geometries in a geometry list-column
+have the same CRS. It may be `NA` in case the CRS is unknown, or
+when we work with local coordinate systems (e.g. inside a building,
+a body, or an abstract space); in that case coordinates are assumed
+to be Cartesian (in case of 2D: defining positions on in a flat
+plane).
+
+A `crs` object contains two character fields: `input` for the
+name (if existing) or the user-definition of the CRS, and `wkt`
+for the WKT-2 specification; WKT-2 is a standard encoding for
+describing CRS that is used throughout the spatial data science
+industry.  When defining a CRS, a PROJ string may be used that is
 understood by the [PROJ](https://proj4.org/) library. It defines
 projection types and (often) defines parameter values for particular
 projections, and hence can cover an infinite amount of different
-projections.  This library (also used by GDAL) provides functions
-to convert or transform between different CRS.  `epsg` is the
-integer ID for a particular, known CRS that can be resolved into a
-`proj4string`. Some `proj4string` values can resolved back into
-their corresponding `epsg` ID, but this does not always work.
-
-The importance of having `epsg` values stored with data besides
-`proj4string` values is that the `epsg` refers to particular,
-well-known CRS, whose parameters may change (improve) over time;
-fixing only the `proj4string` may remove the possibility to benefit
-from such improvements, and limit some of the provenance of datasets,
-but may help reproducibility.
-
-Coordinate reference system transformations can be carried out using
+projections. Alternatively, codes like `EPSG:3035` or `OGC:CRS84`
+may be used; these are well-known identifiers of CRS defined in
+the PROJ database.
+
+Coordinate reference system transformations are carried out using
 `st_transform()`, e.g. converting longitudes/latitudes in NAD27 to
 web mercator (EPSG:3857) can be done by:
 
 ```{r}
-nc.web_mercator <- st_transform(nc, 3857)
+nc.web_mercator <- st_transform(nc, "EPSG:3857")
 st_geometry(nc.web_mercator)[[4]][[2]][[1]][1:3,]
 ```