Index a point
Map a longitude/latitude coordinate to a cell at a given resolution
(0–30). Higher resolutions produce smaller cells.
cell <- a5_lonlat_to_cell(-3.19, 55.95, resolution = 10)
cell
#> <a5_cell[1]>
#> [1] 6344be8000000000
Convert back to the cell centre point:
a5_cell_to_lonlat(cell)
#> <wk_xy[1] with CRS=OGC:CRS84>
#> [1] (-3.183746 55.9806)
Cell boundaries
Get the boundary polygon for one or more cells:
boundary <- a5_cell_to_boundary(cell)
boundary
#> <wk_wkb[1] with CRS=OGC:CRS84>
#> [1] <POLYGON ((-3.175718 55.93546, -3.145905 55.97569, -3.151641 56.01921, -3.219413 56.00818, -3.226037 55.96443, -3.175718 55.93546...>
plot(boundary, col = "#206ead20", border = "#206ead", asp = 1)
Boundaries are returned as wk_wkb vectors by default
(set format = "wkt" for WKT). Both integrate directly with
sf, terra, and other spatial tooling via the wk package.
Hierarchy and compaction
A5 is a hierarchical grid: every cell has a parent
at a coarser resolution and 4 children at the next
finer resolution.
parent <- a5_cell_to_parent(cell)
parent
#> <a5_cell[1]>
#> [1] 6344be0000000000
children <- a5_cell_to_children(cell)
children
#> <a5_cell[4]>
#> [1] 6344be2000000000 6344be6000000000 6344bea000000000 6344bee000000000
We can visualise the relationship - the parent (dark outline)
contains our cell (blue fill), which in turn contains its 4 children
(orange):
plot(NULL, xlim = c(-3.23, -3), ylim = c(55.98, 55.99),
xlab = "", ylab = "", asp = 1
)
plot(a5_cell_to_boundary(a5_cell_to_children(cell)),
col = "#ad6e2020", border = "#ad6e20", add = TRUE)
plot(a5_cell_to_boundary(cell), col = "#206ead40", border = "#206ead",
lwd = 2, add = TRUE)
plot(a5_cell_to_boundary(parent), border = "#333333", lwd = 2, add = TRUE)
Cell area decreases geometrically — each level is roughly 4x
smaller:
a5_cell_area(0:5)
#> Units: [m^2]
#> [1] 4.250547e+13 8.501094e+12 2.125273e+12 5.313184e+11 1.328296e+11
#> [6] 3.320740e+10
Compact and uncompact
When a complete set of siblings is present, a5_compact()
merges them back into their shared parent. This is the inverse of
a5_cell_to_children() and is useful for reducing the size
of large cell sets without losing coverage.
children
#> <a5_cell[4]>
#> [1] 6344be2000000000 6344be6000000000 6344bea000000000 6344bee000000000
a5_compact(children)
#> <a5_cell[1]>
#> [1] 6344be8000000000
# round-trips back to the original
a5_uncompact(a5_compact(children), resolution = 11)
#> <a5_cell[4]>
#> [1] 6344be2000000000 6344be6000000000 6344bea000000000 6344bee000000000
Many a5R functions return compacted output automatically. For
example, a5_grid_disk() and a5_spherical_cap()
compact their results — use a5_uncompact() when you need a
uniform-resolution grid (see Traversal
below).
Traversal
Find neighbouring cells by hop count with
a5_grid_disk(), or by great-circle distance with
a5_spherical_cap():
disk <- a5_grid_disk(cell, k = 10)
cap <- a5_spherical_cap(cell, radius = 50000)
plot(a5_cell_to_boundary(cap), col = "#6ead2020", border = "#6ead20", asp = 1)
plot(a5_cell_to_boundary(disk), col = "#206ead20", border = "#206ead", asp = 1)
Both functions return a compacted cell vector —
sibling groups are merged into coarser parent cells to save space. To
recover a uniform grid at the original resolution, pass the result
through a5_uncompact():
disk_grid <- a5_uncompact(disk, resolution = a5_get_resolution(cell))
plot(a5_cell_to_boundary(disk_grid), col = "#206ead20", border = "#206ead", asp = 1)
Grid generation
a5_grid() is a convenience function provided by a5R (not
part of the underlying a5 Rust crate) that returns all cells at a target
resolution covering a given area — handy for binning, zonal statistics,
and other spatial analysis workflows common in R.
Pass a bounding box as a numeric vector:
cells <- a5_grid(c(-3.3, 55.9, -3.1, 56.0), resolution = 12)
length(cells)
#> [1] 90
plot(a5_cell_to_boundary(cells), col = "#206ead20", border = "#206ead", asp = 1)
Any geometry that wk can handle works too — polygons, sf objects, or
even a5_cell vectors:
library(sf)
#> Linking to GEOS 3.12.2, GDAL 3.12.1, PROJ 9.4.1; sf_use_s2() is TRUE
demo(nc, ask = FALSE, echo = FALSE)
nca5 <- a5_grid(nc, resolution = 9)
plot(a5_cell_to_boundary(nca5), col = "#6d20ad20", border = "#6d20adff", asp = 1)
