Geohash

Geohash is a public domain geocode system invented in 2008 by Gustavo Niemeyer which encodes a geographic location into a short string of letters and digits. Similar ideas were introduced by G.M. Morton in 1966. It is a hierarchical spatial data structure which subdivides space into buckets of grid shape, which is one of the many applications of what is known as a Z-order curve, and generally space-filling curves. Geohashes offer properties like arbitrary precision and the possibility of gradually removing characters from the end of the code to reduce its size (and gradually lose precision). Geohashing guarantees that the longer a shared prefix between two geohashes is, the spatially closer they are together. The reverse of this is not guaranteed, as two points can be very close but have a short or no shared prefix.

History The core part of the Geohash algorithm and the first initiative to similar solution was documented in a report of G.M. Morton in 1966, "A Computer Oriented Geodetic Data Base and a New Technique in File Sequencing". The Morton work was used for efficient implementations of Z-order curve, like in this modern (2014) Geohash-integer version (based on directly interleaving 64-bit integers), but his geocode proposal was not human-readable and was not popular. Apparently, in the late 2000s, G. Niemeyer still didn't know about Morton's work, and reinvented it, adding the use of base32 representation. In February 2008, together with the announcement of the system, he launched the website geohash.org, which allows users to convert geographic coordinates to short URLs which uniquely identify positions on the Earth, so that referencing them in emails, forums, and websites is more convenient. Many variations have been developed, including OpenStreetMap's short link (using base64 instead of base32) in 2009, the 64-bit Geohash in 2014, the exotic Hilbert-Geohash in 2016, and others.

Typical and main usages To obtain the Geohash, the user provides an address to be geocoded, or latitude and longitude coordinates, in a single input box (most commonly used formats for latitude and longitude pairs are accepted), and performs the request. Besides showing the latitude and longitude corresponding to the given Geohash, users who navigate to a Geohash at geohash.org are also presented with an embedded map, and may download a GPX file, or transfer the waypoint directly to certain GPS receivers. Links are also provided to external sites that may provide further details around the specified location. For example, the coordinate pair 57.64911,10.40744 (near the tip of the peninsula of Jutland, Denmark) produces a slightly shorter hash of u4pruydqqvj. The main usages of Geohashes are:

As a unique identifier. To represent point data, e.g. in databases. Geohashes have also been proposed to be used for geotagging. When used in a database, the structure of geohashed data has two advantages. First, data indexed by geohash will have all points for a given rectangular area in contiguous slices (the number of slices depends on the precision required and the presence of geohash "fault lines"). This is especially useful in database systems where queries on a single index are much easier or faster than multiple-index queries. Second, this index structure can be used for a quick-and-dirty proximity search: the closest points are often among the closest geohashes.

Technical description A formal description for computational and mathematical views.

Textual representation For exact latitude and longitude translations Geohash is a spatial index of base 4, because it transforms the continuous latitude and longitude space coordinates into a hierarchical discrete grid, using a recurrent four-partition of the space. To be a compact code it uses base 32 and represents its values by the following alphabet, that is the "standard textual representation".

The "Geohash alphabet" (32ghs) uses all digits 0-9 and all lower case letters except "a", "i", "l" and "o". For example, using the table above and the constant B = 32 {\displaystyle B=32} , the Geohash ezs42 can be converted to a decimal representation by ordinary positional notation:

[ezs42]32ghs = [ ( e × B 4 ) + ( z × B 3 ) + ( s × B 2 ) + ( 4 × B 1 ) + ( 2 × B 0 ) ] 32 g h s {\displaystyle [(e\times B^{4})+(z\times B^{3})+(s\times B^{2})+(4\times B^{1})+(2\times B^{0})]_{32ghs}}

= [ e ] 32 g h s × B 4 + [ z ] 32 g h s × B 3 + [ s ] 32 g h s × B 2 + [ 4 ] 32 g h s × B 1 + [ 2 ] 32 g h s × B 0 {\displaystyle [e]_{32ghs}\times B^{4}+[z]_{32ghs}\times B^{3}+[s]_{32ghs}\times B^{2}+[4]_{32ghs}\times B^{1}+[2]_{32ghs}\times B^{0}}

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