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Note
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5.0 introduces a LatLonPoint type and support for geohash_cell queries
has been removed.
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Geohashes are a way of encoding
lat/lon points as strings. The original intention was to have a
URL-friendly way of specifying geolocations, but geohashes have turned out to
be a useful way of indexing geo-points and geo-shapes in databases.
Geohashes divide the world into a grid of 32 cells—4 rows and 8 columns—each represented by a letter or number. The g cell covers half of
Greenland, all of Iceland, and most of Great Britian. Each cell can be further
divided into another 32 cells, which can be divided into another 32 cells,
and so on. The gc cell covers Ireland and England, gcp covers most of
London and part of Southern England, and gcpuuz94k is the entrance to
Buckingham Palace, accurate to about 5 meters.
In other words, the longer the geohash string, the more accurate it is. If
two geohashes share a prefix— and gcpuuz—then it implies that
they are near each other. The longer the shared prefix, the closer they
are.
That said, two locations that are right next to each other may have completely
different geohashes. For instance, the
Millenium Dome in London has
geohash u10hbp, because it falls into the u cell, the next top-level cell
to the east of the g cell.
Geo-points can index their associated geohashes automatically, but more
important, they can also index all geohash prefixes. Indexing the location
of the entrance to Buckingham Palace—latitude 51.501568 and longitude
-0.141257—would index all of the geohashes listed in the following table,
along with the approximate dimensions of each geohash cell:
| Geohash | Level | Dimensions |
|---|---|---|
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~ 5,004km x 5,004km |
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~ 1,251km x 625km |
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~ 156km x 156km |
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~ 39km x 19.5km |
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~ 4.9km x 4.9km |
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~ 1.2km x 0.61km |
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~ 152.8m x 152.8m |
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~ 38.2m x 19.1m |
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~ 4.78m x 4.78m |
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~ 1.19m x 0.60m |
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~ 14.9cm x 14.9cm |
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~ 3.7cm x 1.8cm |
The geohash_cell filter can use these geohash prefixes
to find locations near a specified lat/lon point.