flexurba (version 0.2.3)

get_patches: Detect patches of cells

Description

The function detects patches of cells based on rook contiguity (horizontal and vertical neighbours) or queen contiguity (horizontal, vertical and diagonal neighbours).

Patches of cells are groups of cells that are surrounded by NA of NaN values. The function identifies patches by converting the SpatRaster into polygons with the functions geos::as_geos_geometry() and geos::geos_unnest(). During this conversion polygons are automatically created as cells with rook contiguity. Touching polygons are afterwards dissolved to create patches with queen contiguity.

The function is similar as terra::patches(), but is faster for large SpatRasters (see details).

Usage

get_patches(x, directions, cells = "all")

Value

SpatRaster with patches of cells. The value of the cells represent the id of the patches.

Arguments

x

SpatRaster

directions

integer. Which cells are considered adjacent: 4 for rooks case (horizontal and vertical neighbours) or 8 for queens case (horizontal, vertical and diagonal neighbours)

cells

character / vector. Either "all" or a vector with specific cell values. If cells="all", patches are identified based on all non-NA cells in x. Otherwise, patches are only identified for cells in x with the specific cell values.

Details

The function is similar as terra::patches(), but is faster for large SpatRasters.

Rook contiguity

Size SpatRaster	terra::patches(directions=4)	flexurba::get_patches(directions=4)
4000 x 4000 (106 642 not-NA cells)	75.157 s	5.351 s
4000 x 4000 (423 888 not-NA cells)	324.846 s	8.336 s

Queen contiguity

Size SpatRaster	terra::patches(directions=8)	flexurba::get_patches(directions=8)
4000 x 4000 (106 642 not-NA cells)	37.322 s	7.737 s
4000 x 4000 (423 888 not-NA cells)	183.428 s	24.094 s

Examples

r <- terra::rast(nrows = 8, ncols = 8, vals = c(
  2, 2, NA, NA, NA, NA, NA, NA,
  NA, NA, NA, NA, NA, NA, NA, NA,
  NA, NA, 1, NA, NA, NA, NA, NA,
  NA, NA, NA, 1, NA, NA, NA, NA,
  NA, NA, NA, NA, 1, 1, 1, 1,
  NA, NA, NA, NA, NA, 1, 1, 1,
  2, NA, NA, NA, NA, NA, NA, NA,
  NA, 2, NA, NA, NA, NA, NA, NA
))
terra::plot(r)
patches_rook1 <- get_patches(r, directions = 4)
terra::plot(patches_rook1)
patches_rook2 <- get_patches(r, directions = 4, cells = 1)
terra::plot(patches_rook2)
patches_queen1 <- get_patches(r, directions = 8)
terra::plot(patches_queen1)
patches_queen2 <- get_patches(r, directions = 8, cells = 1)
terra::plot(patches_queen2)