Build a grid of squares over an extent

Returns the cells of a square grid covering the bounding box of x, as polygons, as the points at their centres, or as the points at their corners. This matches sf::st_make_grid() for square grids.

Usage

ga_make_grid(
  x = NULL,
  cellsize = NULL,
  offset = NULL,
  n = NULL,
  crs = NULL,
  what = "polygons"
)

Arguments

x

a GeoArrow geometry array whose bounding box the grid covers, or NULL when you give cellsize, offset, and n

cellsize

the width and height of a cell; length 1 or 2

offset

the x and y of the lower left corner of the grid; length 2

n

the number of cells across and up; length 1 or 2

crs

a coordinate reference system to record, or NULL for the CRS of x

what

one of "polygons", "centers", or "corners"

Value

a GeoArrow polygon array of the cells, or a point array of their centres or corners

Details

ga_make_grid() starts the grid at offset, the lower left corner of x unless you give it, and steps cellsize across and up. Without cellsize, it divides the extent of x into n cells each way, n being 10 by 10 unless you give it. Without n, it adds cells until the grid covers x. With all of cellsize, offset, and n, it needs no x, and with none of them and no x it returns the 10 degree grid over the globe that sf does.

ga_make_grid() orders the cells along x first, then up in y, and starts each square at its lower left corner, as sf does. It records the CRS of x unless you give crs.

ga_make_grid() builds only square grids. It has no hexagonal grid, which sf::st_make_grid(square = FALSE) gives.

See also

Other construct: ga_linestring(), ga_make_envelope(), ga_make_line(), ga_make_line_agg(), ga_make_polygon(), ga_multilinestring(), ga_multipoint(), ga_multipolygon(), ga_point(), ga_polygon()

Examples

nc <- as.data.frame(read_shapefile(
  system.file("shape/nc.shp", package = "sf")
))

# a 4 by 2 grid of cells over the state
cells <- ga_make_grid(nc$geometry, n = c(4, 2))
sf::st_as_sfc(geoarrow::as_geoarrow_vctr(cells))
#> Geometry set for 8 features 
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> Geodetic CRS:  NAD27
#> First 5 geometries:
#> POLYGON ((-84.32385 33.88199, -82.10713 33.8819...
#> POLYGON ((-82.10713 33.88199, -79.89042 33.8819...
#> POLYGON ((-79.89042 33.88199, -77.6737 33.88199...
#> POLYGON ((-77.6737 33.88199, -75.45698 33.88199...
#> POLYGON ((-84.32385 35.23582, -82.10713 35.2358...

# half degree cells, and the point at the centre of each
centres <- ga_make_grid(nc$geometry, cellsize = 0.5, what = "centers")
centres$length
#> [1] 108