Title: Find the Pole of Inaccessibility (Visual Center) of a Polygon
Version: 0.3.0
Description: A wrapper around the C++ library 'polylabel' from 'Mapbox', providing an efficient routine for finding the approximate pole of inaccessibility of a polygon, which usually serves as an excellent candidate for labeling of a polygon.
License: MIT + file LICENSE
Copyright: see file COPYRIGHTS
Encoding: UTF-8
URL: https://github.com/jolars/polylabelr, https://jolars.github.io/polylabelr/
BugReports: https://github.com/jolars/polylabelr/issues
Depends: R (≥ 3.3.0)
LinkingTo: Rcpp
Imports: Rcpp
RoxygenNote: 7.3.2
Suggests: covr, testthat, spelling, sf
Language: en-US
NeedsCompilation: yes
Packaged: 2024-11-19 18:06:02 UTC; jola
Author: Johan Larsson ORCID iD [aut, cre], Kent Johnson [ctb], Mapbox [cph] (polylabel, variant, and geometry libraries)
Maintainer: Johan Larsson <johanlarsson@outlook.com>
Repository: CRAN
Date/Publication: 2024-11-19 18:30:02 UTC

polylabelr: Find the Pole of Inaccessibility (Visual Center) of a Polygon

Description

A wrapper around the C++ library 'polylabel' from 'Mapbox', providing an efficient routine for finding the approximate pole of inaccessibility of a polygon, which usually serves as an excellent candidate for labeling of a polygon.

Author(s)

Maintainer: Johan Larsson johanlarsson@outlook.com (ORCID)

Other contributors:

See Also

Useful links:

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)

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