Emulate arcpy Union_analysis

Emulate arcpy Union_analysis

st_union_analysis(x, y, union_by = NULL)

Arguments

x, y

sf polygon objects

union_by

optional character string (length 1) specifying the column to union by

Note

sf::st_union modifies geometries and does retain attributes, and for large datasets, avoiding Cartesian joins is essential to minimize RAM use.

Examples

nc <- system.file("shape/nc.shp", package = "sf") |>
  sf::st_read(quiet = TRUE) |>
  dplyr::select(NAME) |>
  sf::st_transform("+proj=laea +lat_0=30 +lon_0=-95")

x <- nc[sample(1:nrow(nc), 40), ] |>
  dplyr::mutate(seral = "old", .before = "geometry") |>
  dplyr::group_by(seral) |>
  dplyr::summarise() |>
  sf::st_cast("POLYGON", warn = FALSE) |>
  sf::st_set_agr("constant")

y <- nc[sample(1:nrow(nc), 30), ] |>
  dplyr::mutate(seral = "mature", .before = "geometry") |>
  dplyr::group_by(seral) |>
  dplyr::summarise() |>
  sf::st_buffer(-5000) |>
  sf::st_cast("POLYGON", warn = FALSE) |>
  sf::st_set_agr("constant")

ggplot2::ggplot() +
  ggplot2::geom_sf(data = x, fill = "orange", alpha = 0.5) +
  ggplot2::geom_sf(data = y, fill = "darkgreen", alpha = 0.5)


## uses attribute values from x for intersections
xy <- sf::st_intersection(x, y) |>
  sf::st_make_valid() |>
  dplyr::select(-dplyr::ends_with(".1"))

dx <- sf::st_difference(x, sf::st_union(y))
dy <- sf::st_difference(y, sf::st_union(x))

u <- st_union_analysis(x, y)

ggplot2::ggplot() +
  ggplot2::geom_sf(data = dx, fill = "lightblue", alpha = 0.25) +
  ggplot2::geom_sf(data = dy, fill = "darkred", alpha = 0.25) +
  ggplot2::geom_sf(data = xy, fill = "purple", alpha = 0.25) +
  ggplot2::geom_sf(data = u, fill = NA, col = "black", alpha = 0.25, linewidth = 1.2)