feat: add mf_get_borders() to extract borders from contiguous polygons

NAMESPACE

@@ -8,6 +8,7 @@ export(mf_choro)
 export(mf_credits)
 export(mf_distr)
 export(mf_export)
+export(mf_get_borders)
 export(mf_get_breaks)
 export(mf_get_leg_pos)
 export(mf_get_links)

R/mf_get_borders.R

@@ -0,0 +1,31 @@
+#' @title Get a border layer from polygons
+#' @description This function extracts borders between contiguous polygons.
+#' @name mf_get_borders
+#' @param x an sf object of POLYGONS, using a projected CRS
+#' @note
+#' If the polygon layer contains topology errors (such as contiguous
+#' polygons not sharing exactly the same boundary) the function may not return
+#' all boundaries correctly. It is possible to use `st_snap()` or other
+#' functions to try and correct these errors.
+#' @return An sf object (MULTILINESTRING) of borders is returned.
+#' @md
+#' @examples
+#' mtq <- mf_get_mtq()
+#' mtq_b <- mf_get_borders(mtq)
+#' mf_map(mtq)
+#' mf_map(mtq_b, col = 1:5, lwd = 4, add = TRUE)
+#' @export
+mf_get_borders <- function(x) {
+  if (get_geom_type(x) != "POLYGON") {
+    stop(paste0('"x" should be a POLYGON sf object'), call. = FALSE)
+  }
+  if (sf::st_is_longlat(x)) {
+    stop("This feature does not work on unprojected (long/lat) layers.",
+         call. = FALSE)
+  }
+  oag <- sf::st_agr(x)
+  x <- sf::st_set_agr(x, "constant")
+  border <- sf::st_collection_extract(sf::st_intersection(x, x), "LINESTRING")
+  border <- sf::st_set_agr(border, c(oag, oag))
+  return(border)
+}

R/mf_get_pencil.R

@@ -1,4 +1,4 @@
-#' @title Pencil Layer
+#' @title Get a pencil layer from polygons
 #' @name mf_get_pencil
 #' @description Create a pencil layer. This function transforms a POLYGON or
 #' MULTIPOLYGON sf object into a MULTILINESTRING one.

R/package.R

@@ -43,6 +43,8 @@
 #' - [mf_get_breaks()] Get class intervals
 #' - [mf_get_mtq()] Get the 'mtq' dataset
 #' - [mf_get_ratio()] Get map width and height values
+#' - [mf_get_pencil()] Get a pencil layer from polygons
+#' - [mf_get_borders()] Get a border layer from polygons
 #
 #' @md
 #'

inst/tinytest/test_get_borders.R

@@ -0,0 +1,7 @@
+mtq <- mf_get_mtq()
+expect_silent(mf_get_borders(mtq))
+expect_error(mf_get_borders(st_transform(mtq, 4326)))
+expect_error(mf_get_borders(st_cast(mtq, "MULTIPOINT")))
+x <- mf_get_borders(mtq)
+expect_equal(nrow(x), 142)
+expect_inherits(sf::st_geometry(x), "sfc_MULTILINESTRING")

pkgdown/_pkgdown.yml

@@ -55,6 +55,7 @@ reference:
     - mf_get_mtq
     - mf_get_ratio
     - mf_get_pencil
+    - mf_get_borders
 
 figures:
   dev: grDevices::png