a5_cell_to_boundary_xy: Get cell boundary vertices as flat coordinate vectors

In a5R: 'A5' Discrete Global Grid System

Get cell boundary vertices as flat coordinate vectors

Description

Returns the boundary vertices of every cell in one pair of coordinate vectors, with a vector of per-cell vertex counts. This skips building a geometry for each cell, so it suits drawing many cells at once, for example as a single grid::polygonGrob() with id.lengths = lengths.

Usage

a5_cell_to_boundary_xy(cell, closed_ring = FALSE, segments = NULL, closed)

Arguments

cell	An a5_cell vector.
closed_ring	Logical scalar; if TRUE each ring repeats its first vertex at the end. Defaults to FALSE, unlike a5_cell_to_boundary(), since polygon drawing closes rings itself.
segments	Integer scalar or NULL. Number of segments per cell edge. A5 cell edges are slightly curved on the sphere: a single segment between two vertices strays from the true edge by up to about 450 m at resolution 6, 25 m at resolution 8 and 1.5 m at resolution 10. The error falls with the square of the number of segments. NULL uses the A5 default of max(1, 2^(6 - resolution)), so at resolution 6 and finer each edge is a single segment. Pass a larger value when the polygons are used for spatial tests rather than display.
closed	Deprecated: renamed closed_ring, as in the other A5 libraries.

Details

The vertices and their order are those of a5_cell_to_boundary(). Longitudes within a ring are continuous, as in the other A5 libraries: a ring that crosses the antimeridian continues past 180 or -180 instead of jumping, so x can fall outside [-180, 180]. Shift or split such rings before drawing them on a map that wraps at the antimeridian. A ring around a pole cannot be drawn as a simple polygon in longitude and latitude.

Value

A list with three elements:

• x: double vector of longitudes, every cell's vertices in turn.

• y: double vector of latitudes, aligned with x.

• lengths: integer vector with the number of vertices of each cell, the same length as cell. An NA cell has length 0.

See Also

a5_cell_to_boundary() for polygon geometries.

Examples

cells <- a5_lonlat_to_cell(c(-3.19, -3.10), c(55.95, 55.90), resolution = 9)
xy <- a5_cell_to_boundary_xy(cells)
xy$lengths

# one grob for all cells
grob <- grid::polygonGrob(
  xy$x, xy$y,
  id.lengths = xy$lengths,
  default.units = "native"
)

a5R documentation built on Oct. 19, 2026, 5:08 p.m.