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Read a .poly file.

Source: R/read-poly.R
read_poly.Rd

Read a .poly file.

Usage

read_poly(input, crs = "OGC:CRS84", ...)

Arguments

input

Character vector representing a polygon object saved using the .poly format. Can be also a path to a file or a URL pointing to a valid .poly file.

crs

The Coordinate Reference System (CRS) of the input polygon.

...

Further arguments passed to readLines() (which is the function used to read external .poly files).

Value

A sfc_MULTIPOLYGON/sfc object.

Details

The Polygon Filter File Format (.poly) is defined here. The code behind the function was inspired by the parse_poly function defined here.

Geofabrik stores the .poly files used to generate their extracts. Furthermore, a nice collection of exact-border poly files created from cities with an OSM Relation ID is available in this git repository on github: https://github.com/jameschevalier/cities.

The default value for the crs argument is "OGC:CRS84" instead of "4326" or "EPSG:4326" since, by definition, the coordinates are provided as "longitude, latitude" (but these differences should be relevant only when sf::st_axis_order() is TRUE).

Examples

toy_poly <- c(
  "test_poly",
  "first_area",
  "0 0",
  "0 1",
  "1 1",
  "1 0",
  "0 0",
  "END",
  "END"
)
(out <- read_poly(toy_poly))
#> Geometry set for 1 feature 
#> Geometry type: MULTIPOLYGON
#> Dimension:     XY
#> Bounding box:  xmin: 0 ymin: 0 xmax: 1 ymax: 1
#> Geodetic CRS:  WGS 84
#> MULTIPOLYGON (((0 0, 0 1, 1 1, 1 0, 0 0)))
plot(out)


if (FALSE) {
italy_poly <- "https://download.geofabrik.de/europe/italy.poly"
plot(read_poly(italy_poly))}