Get cell boundary vertices as flat coordinate vectors

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 = FALSE, segments = NULL)

Arguments

cell

An a5_cell vector.

closed

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.

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.

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.

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
#> [1] 5 5

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