Merges child cells into their parent when all children are present, so that the same set of cells is named by fewer of them.
Arguments
- cell_ids
Cell IDs to compact. For H3 grids, a character vector. For ISEA grids, a character vector of hierarchical index strings, as
cell_to_indexreturns for apertures 3, 4 and 7.- grid
A HexGridInfo object specifying the grid.
Value
A character vector of compacted cell IDs. Cells that could be merged into parents appear as parent IDs at coarser resolution.
Details
H3 backend: Uses the vendored H3 compactCells function.
ISEA backend: Cells are grouped by get_parent(), and a group holding as
many distinct cells as its parent has children replaces them, from the finest
resolution up until no further compaction is possible. That count is the
aperture away from the twelve icosahedron vertices and fewer at one, and it
is read rather than assumed: a vertex cell sits at the corner of several
quads and carries an index spelling in each, so appending a digit to any one
spelling names neither all of its children nor only its children.
What is preserved is the set of cells: uncompacting the result at the original resolution returns the input. The covered area is not identical, because a hexagonal hierarchy is not congruent at any aperture – a parent does not tile exactly into its children – so an area computed on the compacted set differs from one computed on the original.
See also
hex_uncompact() for the inverse operation,
get_parent(), get_children() for hierarchical operations
Examples
# \donttest{
# H3 compaction
g <- hex_grid(resolution = 3, type = "h3")
parent <- "832830fffffffff"
children <- get_children(parent, g)[[1]]
compact <- hex_compact(children, g)
compact # Should return the parent
# }
# ISEA, on the default aperture
g <- hex_grid(resolution = 2, aperture = 3)
children <- cell_to_index(get_children(40L, g)[[1]],
hex_grid(resolution = 3, aperture = 3))
hex_compact(children, g)