use native datum for default graticule, rather than the OGC:CRS84

Author: edzer

NEWS.md

@@ -1,5 +1,7 @@
 # version 1.0-24
 
+* `st_graticule()` creates graticule by default in the geographic coordinate reference system of arguments `x` or `crs`, and only if nothing is found or provided in `OGC:CRS84`
+
 * the figure margins parameter `mar` can be specified in a call to `plot.sf()`; #2558
 
 * fix class label setting in `[.sf()`; #2557

R/crs.R

@@ -98,11 +98,11 @@ fix_crs = function(x) {
 
 
 #' @name st_crs
-#' @param parameters logical; \code{FALSE} by default; if \code{TRUE} return a list of coordinate reference system parameters, with named elements \code{SemiMajor}, \code{InvFlattening}, \code{units_gdal}, \code{IsVertical}, \code{WktPretty}, and \code{Wkt}
+#' @param parameters logical; \code{FALSE} by default; only for the `st_crs.sfc()` method: if \code{TRUE} return a classed list of coordinate reference system parameters, with named elements \code{SemiMajor}, \code{InvFlattening}, \code{units_gdal}, \code{IsVertical}, \code{WktPretty}, \code{Wkt}, \code{Name}, \code{proj4string}, \code{epsg}, \code{yx}, \code{ProjJson}, \code{WKT1_ESRI}, \code{srid} (in authority:code form), \code{axes} (a data.frame with columns \code{name} and \code{orientation}), \code{gcs_crs} with the WKT of the base geographic coordinate system, \code{ud_unit}. The list has class \code{crs_parameters}.
 #' @export
 st_crs.sfc = function(x, ..., parameters = FALSE) {
 	crs = fix_crs(attr(x, "crs"))
-	if (parameters) {
+	if (isTRUE(parameters)) {
 		p = crs_parameters(crs)
 		p$ud_unit = crs_ud_unit(crs)
 		p

R/graticule.R

@@ -1,3 +1,15 @@
+st_datum = function(x) {
+	stopifnot(inherits(x, c("crs", "sf", "sfc")))
+	if (inherits(x, "crs"))
+		x = st_sfc(st_point(), crs = x)
+	x = st_geometry(x) # in case inherits(x, "sf")
+	datum = st_crs(st_crs(x, parameters = TRUE)$gcs_crs)
+	if (is.na(datum))
+		st_crs('OGC:CRS84') # + message or warning?
+	else
+		datum
+}
+
 #' Compute graticules and their parameters
 #'
 #' Compute graticules and their parameters
@@ -15,7 +27,7 @@
 #' @export
 #' @param x object of class \code{sf}, \code{sfc} or \code{sfg} or numeric vector with bounding box given as (minx, miny, maxx, maxy).
 #' @param crs object of class \code{crs}, with the display coordinate reference system
-#' @param datum either an object of class \code{crs} with the coordinate reference system for the graticules, or \code{NULL} in which case a grid in the coordinate system of \code{x} is drawn, or \code{NA}, in which case an empty \code{sf} object is returned.
+#' @param datum either an object of class \code{crs} with the coordinate reference system for the graticules, or \code{NULL} in which case a grid in the coordinate system of \code{x} is drawn, or \code{NA}, in which case an empty \code{sf} object is returned. If missing and \code{x} has a crs with a datum, the geographic coordinate system (datum) of \code{x} is taken.
 #' @param lon numeric; values in degrees East for the meridians, associated with \code{datum}
 #' @param lat numeric; values in degrees North for the parallels, associated with \code{datum}
 #' @param ndiscr integer; number of points to discretize a parallel or meridian
@@ -61,7 +73,7 @@
 #' plot(usa, graticule = st_crs(4326), axes = TRUE, lon = seq(-60,-130,by=-10))
 #' }
 st_graticule = function(x = c(-180, -90, 180, 90), crs = st_crs(x),
-	datum = st_crs(4326), ..., lon = NULL, lat = NULL, ndiscr = 100,
+	datum = st_crs('OGC:CRS84'), ..., lon = NULL, lat = NULL, ndiscr = 100,
 	margin = 0.001)
 {
 	s2 = sf_use_s2()
@@ -74,6 +86,11 @@ st_graticule = function(x = c(-180, -90, 180, 90), crs = st_crs(x),
 			lon = seq(-180, 180, by = 20)
 		if (is.null(lat))
 			lat = seq(-80, 80, by = 20)
+	} else if (missing(datum)) {
+		datum = if (inherits(x, c("sf", "sfc")))
+				st_datum(x)
+			else
+				st_datum(crs)
 	}
 
 	if (is.null(crs))

R/plot.R

@@ -659,7 +659,7 @@ plot_sf = function(x, xlim = NULL, ylim = NULL, asp = NA, axes = FALSE, bgc = pa
 	linAxis = function(side, ..., lon, lat, ndiscr, reset, at) axis(side = side, ...)
 	if (!missing(graticule) && !identical(graticule, FALSE)) {
 		g = if (isTRUE(graticule))
-				st_graticule(pl_reg[c(1,3,2,4)], st_crs(x), st_crs('OGC:CRS84'), ...)
+				st_graticule(pl_reg[c(1,3,2,4)], st_crs(x), st_datum(x), ...)
 			else if (inherits(graticule, "crs") && !is.na(graticule))
 				st_graticule(pl_reg[c(1,3,2,4)], st_crs(x), graticule, ...)
 			else