#Addressing

Two ways to turn a coordinate into a key a database can index.

#Geohashes

geo.geohash(geo.point(47.2692, 11.4041), 7);   # "u22hgt0"

Each character narrows the cell a point lies in, alternating longitude and latitude bits, five bits per character. length is 1 to 20; anything else raises. GEOHASH_LENGTH (9) is a sensible default to pass: nine characters give a cell of about 4.8 m by 4.8 m at the equator, six about 1.2 km by 0.6 km.

Nearby points share a prefix, so a prefix scan is a proximity search and truncating a hash coarsens it:

def hash as string init geo.geohash($p, 9);
strings.substring($hash, 0, 5);   # the same as geo.geohash($p, 5)

(testGeohashIsAPrefixCode.)

The alphabet (GEOHASH_ALPHABET) is base 32 without a, i, l and o, so a hash read aloud or copied by hand has no letter to confuse with 1 or 0.

#Cell edges

Two points either side of a cell boundary share no prefix however close they are; across the equator or the prime meridian they differ in the first character. That follows from recursive subdivision, not from this implementation. A prefix scan finds most of what is near a point but not all of it: when that matters, also check the neighbouring cells, or query a bounding box from bounds.

#Decoding

A geohash names a cell, not a point:

def box as geo.Bounds init geo.geohashBounds("u22hgt0");
def middle as geo.Point init geo.geohashDecode("u22hgt0");

geohashDecode returns the centre of the cell, so it is off by up to half a cell; encoding that centre at the same length gives the hash back. Both accept upper case and raise on an empty string or a character outside the alphabet.

#Map tiles

def t as geo.Tile init geo.tileOf(geo.point(47.2692, 11.4041), 14);

That is tile 14/8711/5744 in the Web Mercator (EPSG:3857) scheme that OpenStreetMap, Google Maps and other slippy maps share: at zoom z the world is a grid of 2^z by 2^z tiles, x counting east from the antimeridian and y south from the top.

https://tile.openstreetmap.org/14/8711/5744.png

Zoom runs from 0 to MAX_ZOOM (22); anything else raises. tileBounds returns the latitude and longitude box a tile covers, and raises on an x or y outside the grid. quadkey writes the same address as the single string Bing Maps and some tile caches use, one digit per zoom level:

geo.quadkey(geo.Tile{x: 8711, y: 5744, z: 14});   # "12023002220111"

A parent tile's quadkey is a prefix of its children's, as with geohashes. quadkey checks the zoom only, not x and y.

#The Mercator limit

Web Mercator sends the poles to infinity, so it stops at MERCATOR_LIMIT, 85.0511287798 degrees north and south. tileOf clamps a latitude beyond it instead of raising: a track into the high Arctic lands in the top row. The poles themselves need another projection.

#Which to use

Geohashes suit a key in a store with prefix or range queries, and a short location that can be read out. Tiles suit fetching or caching map imagery and bucketing points at a fixed scale. Neither is a spatial index for exact nearest-neighbour queries.