Package 'polylabelr'

Pole of Inaccessibility (Visual Center) of a Polygon

Description

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

Usage

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

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().

Value

A list with items

x	x coordinate of the center
y	y coordinate of the center
dist	distance to the enclosing polygon

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. doi:10.1080/14702540801897809

https://github.com/mapbox/polylabel

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

See Also

grDevices::xy.coords(), 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))

## Not run: 
# 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)

## End(Not run)

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Pole of Inaccessibility for Multi-Polygons

Description

This function is a convenience function to be used with multi-polygons (lists of polygons). The function simply calls poi() for each polygon in the list and returns the point wit the maximum distance to the boundaries of the enclosing polygon.

Usage

poi_multi(polygons, ...)

Arguments

polygons	a list, each element containing entries x and y. See poi() for details about what these need to be.
...	Arguments passed on to poi 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

Value

A list with items

x	x coordinate of the center
y	y coordinate of the center
dist	distance to the enclosing polygon

See Also

poi()

Examples

p1 <- rbind(
  c(0, 0),
  c(1, 0),
  c(3, 2),
  c(2, 4),
  c(1, 4),
  c(0, 0),
  c(NA, NA),
  c(1, 1),
  c(1, 2),
  c(2, 2),
  c(1, 1)
)
p2 <- rbind(
  c(3, 0),
  c(4, 0),
  c(4, 1),
  c(3, 1),
  c(3, 0),
  c(NA, NA),
  c(3.3, 0.8),
  c(3.8, 0.8),
  c(3.8, 0.3),
  c(3.3, 0.3),
  c(3.3, 0.3)
)
p3 <- rbind(
  c(3, 3),
  c(4, 2),
  c(4, 3),
  c(3, 3)
)
mpol <- list(p1, p2, p3)

plot.new()
plot.window(c(0, 4), c(0, 4), asp = 1)
for (i in seq_along(mpol)) {
  polypath(mpol[[i]])
}

res <- poi_multi(mpol, precision = 1e-5)
points(res, pch = 19, col = "steelblue4")

res

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