Convert cells to sf polygons — st_as_sf.globalhex

Cell boundaries are spherical polygons: DGGRID reports their vertices, and the edges between them are great-circle arcs. That is exactly what sf::st_area() and other spherical (s2) operations expect, but a flat longitude/latitude map draws straight lines between vertices, so cells that cross the antimeridian (180 degrees longitude) would stretch across the whole map and cells containing a pole would draw as twisted shapes.

Usage

# S3 method for class 'globalhex_cell'
st_as_sf(x, wrap = TRUE, ...)

Arguments

x: A globalhex_cell vector.
wrap: Whether to split antimeridian cells, close polar cells, and densify long edges for flat longitude/latitude use.
...: Passed to methods; currently unused.

Value

An sf object in EPSG:4326 with one row and geometry per cell.

Details

With wrap = TRUE (the default) the polygons are prepared for flat longitude/latitude use:

edges longer than one degree are densified along their great circles;
cells crossing the antimeridian are split into a MULTIPOLYGON whose parts touch longitude -180 and 180;
cells containing a pole are closed along the map edge at latitude 90 or -90, so they draw as a polar cap.

Longitudes stay in [-180, 180] and spherical areas are unchanged. When any cell is split, every geometry is returned as a MULTIPOLYGON so the column has a single type. For a Pacific-centred map, use sf::st_shift_longitude() on the result to move longitudes to [0, 360].

Use wrap = FALSE for the raw spherical polygons with one ring of DGGRID vertices per cell, for example before projecting to a polar CRS.

Examples

if (requireNamespace("sf", quietly = TRUE)) {
  seam <- point_to_cell(c(180, 10), 6)
  sf::st_geometry(sf::st_as_sf(seam))
  sf::st_geometry(sf::st_as_sf(seam, wrap = FALSE))
}
#> Geometry set for 1 feature 
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -179.9898 ymin: 9.812935 xmax: 179.9192 ymax: 10.06704
#> Geodetic CRS:  WGS 84
#> POLYGON ((-179.9898 10, 179.916 10.06704, 179.8...