Return the cube face in 0 to 5 an S2 cell sits on

s2GetFace(s2cell) → integer

S2 projects the sphere onto the six faces of a circumscribed cube and lays a Hilbert curve on each, so the face is the first three bits of the identifier and two cells on different faces share no ancestor.

SELECT s2GetFace(s2cell '47c3c3');
-- 2
SELECT s2GetFace(geoToS2Cell(geography 'SRID=4326;Point(-122.4 37.8)', 10));
-- 4

Convert between an S2 cell and its token, the form the S2 libraries publish

s2CellToToken(s2cell) → text
s2TokenToCell(text) → s2cell
ts2CellToToken(ts2cell) → ttext

A token is the identifier in hexadecimal with its trailing zeros removed, so a coarse cell reads shorter than a fine one and the text sorts as the identifier does.

SELECT s2CellToToken(s2cell '47c3c3');
-- 47c3c3
SELECT s2TokenToCell('47c3c3');
-- 47c3c3
SELECT asText(ts2CellToToken(ts2cell '47c3c3@2001-01-01'));
-- "47c3c3"@2001-01-01

Return the level of the deepest cell containing two S2 cells

s2CommonAncestorLevel(s2cell,s2cell) → integer

Two cells on different cube faces are contained by no cell, and the function answers -1 for them.

SELECT s2CommonAncestorLevel(s2cell '47c3c3', s2cell '47c4');
-- 5
SELECT s2CommonAncestorLevel(s2cell '47c3c3', s2cell '808581');
-- -1

Return true if the first S2 cell contains the second one

s2CellContains(s2cell,s2cell) → boolean

SELECT s2CellContains(s2cell '47c4', s2cell '47c3c3');
-- t

Return the first and the last leaf cell an S2 cell contains

s2RangeMin(s2cell) → s2cell
s2RangeMax(s2cell) → s2cell

The descendants of an S2 cell occupy a contiguous interval of the Hilbert curve, so an ancestor test is the pair of comparisons other >= s2RangeMin(cell) and other <= s2RangeMax(cell). That is what S2 brings that a hexagonal or a Web-Mercator grid does not, and it is a filter feeding the stbox index rather than an index key of its own.

SELECT s2RangeMin(s2cell '47c3c3');
-- 47c3c20000000001
SELECT s2RangeMax(s2cell '47c3c3');
-- 47c3c3ffffffffff

Return the descendant of an S2 cell at the given position along the Hilbert curve

s2CellToChild(s2cell,level integer,position integer) → s2cell

The level is exactly one finer than the cell's own, so the position runs from 0 to 3; cellToChildren, which the hierarchy section states, answers every descendant at any finer level.

SELECT s2CellToChild(s2cell '47c3c3', 11, 0);
-- 47c3c24

Return the length in metres of an edge of an S2 cell

s2EdgeLength(s2cell,edge integer) → float

The edge runs from the vertex of the given index, which is 0 to 3, to the next one counterclockwise, and the length is measured on the WGS84 sphere.

SELECT round(s2EdgeLength(s2cell '47c3c3', 0)::numeric, 3);
-- 9270.048

Return the four cells sharing an edge with an S2 cell

s2EdgeNeighbors(s2cell) → s2cellset

An S2 cell is a quadrilateral, so it has four edge neighbours and no ring of a given radius; the H3 grid disk has no S2 analogue.

SELECT s2EdgeNeighbors(s2cell '47c3c3');
-- {"47c3c1", "47c3c5", "47c3dd", "47c3e9"}