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 Britain. 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:

The geohash_cell filter can use these geohash prefixes to find locations near a specified lat/lon point.