Pole of Inaccessibility (Visual Center) of a Polygon

This function computes and returns the approximate pole of inaccessibility for a polygon using a quadtree-based algorithm developed by the people from Mapbox.

poi(x, y = NULL, precision = 1)

Arguments

x	a vector of x coordinates or a matrix or data.frame of x and y coordinates, a list of components x and y, a time series (see `grDevices::xy.coords()` for details), or a simple features object from package `sf`.
y	a vector of y coordinates. Only needs to be provided if `x` is vector.
precision	the precision to use when computing the center

Source

Garcia-Castellanos & Lombardo, 2007. Poles of inaccessibility: A calculation algorithm for the remotest places on earth Scottish Geographical Journal, Volume 123, 3, 227-233. https://dx.doi.org/10.1080/14702540801897809

https://github.com/mapbox/polylabel

https://blog.mapbox.com/a-new-algorithm-for-finding-a-visual-center-of-a-polygon-7c77e6492fbc

Value

A list with items

x

x coordinate of the center

y

y coordinate of the center

dist

distance to the enclosing polygon

Details

If there are any NA values in the input, they will be treated as separators for multiple paths (rings) of the polygon, mimicking the behavior of graphics::polypath().

Examples

plot_path <- function(x, y, ...) {
  plot.new()
  plot.window(range(x, na.rm = TRUE), range(y, na.rm = TRUE))
  polypath(x, y, ...)
}

x <- c(5, 10, 10, 5, 5, 6, 6, 7, 7, 6, 8, 8, 9, 9, 8)
y <- c(5, 5, 10, 10, 5, 6, 7, 7, 6, 6, 8, 9, 9, 8, 8)

plot_path(x, y, col = "grey", border = NA)

points(poi(x, y))

if (FALSE) {
# Find visual centers for North Carolina counties
library(sf)
nc <- st_read(system.file("shape/nc.shp", package="sf"))
locations = do.call(rbind, poi(nc, precision=0.01))
plot(st_geometry(nc))
points(locations)
}