fixes #2406

Author: edzer

NAMESPACE

@@ -217,6 +217,9 @@ S3method(st_difference,sfc)
 S3method(st_difference,sfg)
 S3method(st_drop_geometry,default)
 S3method(st_drop_geometry,sf)
+S3method(st_exterior_ring,sf)
+S3method(st_exterior_ring,sfc)
+S3method(st_exterior_ring,sfg)
 S3method(st_filter,sf)
 S3method(st_geometry,sf)
 S3method(st_geometry,sfc)
@@ -447,6 +450,7 @@ export(st_drivers)
 export(st_drop_geometry)
 export(st_equals)
 export(st_equals_exact)
+export(st_exterior_ring)
 export(st_filter)
 export(st_geometry)
 export(st_geometry_type)

NEWS.md

@@ -1,5 +1,7 @@
 # version 1.0-17
 
+* add `st_exterior_ring()` to extract exterior rings (remove holes); #2406
+
 * add `text.sf()`, `text.sfc()`, `points.sf()`, `points.sfc()` to annotate base plots at geometry centroids; #2399
 
 * `st_sf()` no longer strips `tbl` or `tbl_df` class labels; #2378

R/geom-transformers.R

@@ -1149,3 +1149,29 @@ st_line_interpolate = function(line, dist, normalized = FALSE) {
 	st_sfc(CPL_line_interpolate(recycled[[1]], recycled[[2]], normalized), 
 		   crs = st_crs(line))
 }
+
+#' @export
+#' @name geos_unary
+st_exterior_ring = function(x, ...) UseMethod("st_exterior_ring")
+
+#' @export
+st_exterior_ring.sf = function(x, ...)
+	st_set_geometry(x, st_exterior_ring(st_geometry(x)))
+
+#' @export
+st_exterior_ring.sfg = function(x, ...)
+	st_exterior_ring(st_sfc(x))[[1]]
+
+#' @export
+st_exterior_ring.sfc = function(x, ...) {
+	stopifnot(all(st_dimension(x, NA_if_empty = FALSE) == 2))
+	exterior_sfg = function(x) {
+		if (inherits(x, "MULTIPOLYGON"))
+			st_multipolygon(lapply(st_cast(st_sfc(x), "POLYGON"), exterior_sfg))
+		else if (inherits(x, "POLYGON"))
+			st_polygon(x[1])
+		else
+			stop(paste("no exterior_ring method for objects of class", class(x)[1]))
+	}
+    st_as_sfc(lapply(x, exterior_sfg), crs = st_crs(x))
+}

man/geos_unary.Rd

@@ -340,3 +340,21 @@ out = merge(x, y, all.x=TRUE)
 class(out)
 
 st_as_sf(st_sfc(st_point(0:1)))
+
+# st_exterior_ring():
+outer = matrix(c(0,0,10,0,10,10,0,10,0,0),ncol=2, byrow=TRUE)
+hole1 = matrix(c(1,1,1,2,2,2,2,1,1,1),ncol=2, byrow=TRUE)
+hole2 = matrix(c(5,5,5,6,6,6,6,5,5,5),ncol=2, byrow=TRUE)
+pts = list(outer, hole1, hole2)
+pl1 = st_polygon(pts)
+mpl1 = st_multipolygon(list(pl1,pl1+20))
+
+spl1 = st_as_sfc(list(pl1),crs=4326)
+smpl1 = st_as_sfc(list(mpl1),crs=4326)
+
+st_exterior_ring(spl1[[1]])
+st_exterior_ring(spl1)
+st_exterior_ring(st_sf(a = 1, geom = spl1))
+st_exterior_ring(smpl1[[1]])
+st_exterior_ring(st_sfc(smpl1))
+st_exterior_ring(st_sf(a = 1, geom = st_sfc(smpl1)))

tests/sfc.R

@@ -1,7 +1,7 @@
 
-R version 4.3.3 (2024-02-29) -- "Angel Food Cake"
+R version 4.4.0 (2024-04-24) -- "Puppy Cup"
 Copyright (C) 2024 The R Foundation for Statistical Computing
-Platform: x86_64-pc-linux-gnu (64-bit)
+Platform: x86_64-pc-linux-gnu
 
 R is free software and comes with ABSOLUTELY NO WARRANTY.
 You are welcome to redistribute it under certain conditions.
@@ -1086,6 +1086,52 @@ CRS:           NA
             x
 1 POINT (0 1)
 > 
+> # st_exterior_ring():
+> outer = matrix(c(0,0,10,0,10,10,0,10,0,0),ncol=2, byrow=TRUE)
+> hole1 = matrix(c(1,1,1,2,2,2,2,1,1,1),ncol=2, byrow=TRUE)
+> hole2 = matrix(c(5,5,5,6,6,6,6,5,5,5),ncol=2, byrow=TRUE)
+> pts = list(outer, hole1, hole2)
+> pl1 = st_polygon(pts)
+> mpl1 = st_multipolygon(list(pl1,pl1+20))
+> 
+> spl1 = st_as_sfc(list(pl1),crs=4326)
+> smpl1 = st_as_sfc(list(mpl1),crs=4326)
+> 
+> st_exterior_ring(spl1[[1]])
+POLYGON ((0 0, 10 0, 10 10, 0 10, 0 0))
+> st_exterior_ring(spl1)
+Geometry set for 1 feature 
+Geometry type: POLYGON
+Dimension:     XY
+Bounding box:  xmin: 0 ymin: 0 xmax: 10 ymax: 10
+Geodetic CRS:  WGS 84
+POLYGON ((0 0, 10 0, 10 10, 0 10, 0 0))
+> st_exterior_ring(st_sf(a = 1, geom = spl1))
+Simple feature collection with 1 feature and 1 field
+Geometry type: POLYGON
+Dimension:     XY
+Bounding box:  xmin: 0 ymin: 0 xmax: 10 ymax: 10
+Geodetic CRS:  WGS 84
+  a                           geom
+1 1 POLYGON ((0 0, 10 0, 10 10,...
+> st_exterior_ring(smpl1[[1]])
+MULTIPOLYGON (((0 0, 10 0, 10 10, 0 10, 0 0)), ((20 20, 30 20, 30 30, 20 30, 20 20)))
+> st_exterior_ring(st_sfc(smpl1))
+Geometry set for 1 feature 
+Geometry type: MULTIPOLYGON
+Dimension:     XY
+Bounding box:  xmin: 0 ymin: 0 xmax: 30 ymax: 30
+Geodetic CRS:  WGS 84
+MULTIPOLYGON (((0 0, 10 0, 10 10, 0 10, 0 0)), ...
+> st_exterior_ring(st_sf(a = 1, geom = st_sfc(smpl1)))
+Simple feature collection with 1 feature and 1 field
+Geometry type: MULTIPOLYGON
+Dimension:     XY
+Bounding box:  xmin: 0 ymin: 0 xmax: 30 ymax: 30
+Geodetic CRS:  WGS 84
+  a                           geom
+1 1 MULTIPOLYGON (((0 0, 10 0, ...
+> 
 > proc.time()
    user  system elapsed 
-  5.156   1.454   5.144 
+  5.477   1.381   5.407