tessellatr - Generate quasi-random points or tiles inside sf-POLYGONS

Simple wrappers for spatstat.random::rSSI() to generate quasi-random points or tiles inside sf-POLYGON or MULTIPOLYGON objects. Sometimes, regular random sampling is not enough, and you may want a more uniform sample distribution, where no two points are too close together:

There is a convenient implementation in spatstat.random::rSSI(), but coercion between sf and spatstat-objects is messy, which is what these wrappers handle for you.

Installation

The package is not on CRAN, but you can install it from Github using

devtools::install_github("kssrr/tessellatr")

Alternatively, you can just copy the functions from R/default_generators.R into your session.

Example usage

First, we need a POLYGON or MULTIPOLYGON object:

germany <- rnaturalearth::ne_countries(country = "Germany", returnclass = "sf")

Both st_random_points() and st_random_voronoi() require you to specify a minimum distance between the points for sampling; the unit here is the unit of the CRS of the POLYGON, which you can check with sf::st_crs():

germany  |> 
  sf::st_crs()  |> 
  unclass()  |> 
  _$wkt  |> 
  stringr::str_extract('LENGTHUNIT\\[(.*)\\]')

#> [1] "LENGTHUNIT[\"metre\",1]]]"

So here, the unit is metres. We can now generate the random points, passing the minimum distance in metres:

pts <- tessellatr::st_random_points(germany, min_dist = 20000) # = 20km

Let's get a look at the result:

library(ggplot2)

germany |> 
  ggplot() +
  geom_sf() +
  geom_sf(pts, mapping = aes()) +
  theme_void()

The same works for st_random_voronoi() (here, a minimum distance of 20'000 means that the centroids of the tiles will always be at least 20 kilometres apart.):

vor <- tessellatr::st_random_voronoi(germany, 20000)

vor |> 
  ggplot() +
  geom_sf() +
  theme_void()

You can also specify a fixed number of points to generate; by default, as many points as possible without violating the minimum distance will be placed. For some objects, you may need to turn off spherical geometry: sf::sf_use_s2(FALSE).