Pharo package to decode/encode Geohashes to/from latitude and longitude (geolocation). See http://en.wikipedia.org/wiki/Geohash Geohash can be used for a nerby search when working with thousands of locations.
Convert a given geolocation (latitude, longitude) to a geohash.
theGeolocation := SimGeolocation longitude: 7.853801 latitude: 48.013854.
theGeohash := theGeolocation asGeohash.
The longer the hash is, the more precisely it describes a rectangle on a map. The precision can be selected just as easily. Just search for the distance you want to get the locations (in meter). Then you will receive a Geohash with the needed precision.
theGeolocation := SimGeolocation longitude: 7.853801 latitude: 48.013854.
thePrecisionDigits := SimGeohash precisionForMeter: 2000.
theGeohash := theGeolocation asGeohashWithPrecision: thePrecisionDigits.
Geohash is calculated on the base of 0123456789bcdefghjkmnpqrstuvwxyz. Because of the hashing algorithm not always e.g. Geohash "9q8yd" is not the neightbour of "89q8yc".
The geohash describes a rectangle where the given location is somewhere in between. So for the search of the nearby locations we need to get the neighbour hashes
(SimGeohash new: 'u0t94') neighbours.
After that you receive all the neighbour hashes
u0t93 u0t96 u0t97
u0t91 u0t94 u0t95
u0t8c u0t8f u0t8g
To receive the bounding coordinates of the current geohash you easyly can get those bound. You will need to also receive the center of the bound, so you can easily get the center of the geohash. Indeed this will be performed if you decode a heohash to get a geolocation
theGeobound := (SimGeohash new: 'u0t94') bound.
theGeolocation := theGeobound center
(SimGeohash new: 'u0t94') asGeolocation = theGeolocation
This will result into 48.01, 7.844
The geohash is quite complex to understand, but each letter is the 5 bit result of a calculation. To get the binary representation you can get this representation.
(SimGeohash new: 'u0t94') asBinaryNumberString = '1101000000110010100100100'
If you will do the math you can see that
00000 => 0
11001 => t
01001 => 9
00100 => 4
Its useful to get all ranges between 2 geohases.
SimGeohash geohashesBetween: (SimGeohash new: 'u0t92') and: (SimGeohash new: 'u0t94')
=> Results in Geohashes: u0t92, u0t93, u0t94
To find the nearest locations to a given geolocation, you first get the geohash for the specific location; afterwards all neighbour geohashes. Because of the reason that nearby characters are not always the neighbour geohash in the worst case you have to do 9 queries against the database. This can be improved by searching for ranges. Instead of having to make 9 requests, we can reduce the number of requests to normally 3-5 with searchranges. TO stay in the example of the neighbours on the neighbour-section
theSearchRanges := (SimGeohash new: 'u0t94') searchRanges.
(SimGeohash new: 'u0t94') searchRanges
u0t8c -> u0t8d
u0t8f -> u0t8h
u0t91 -> u0t98
You will end up having some more geohashes in the resultSet, but therefore you have decreased the query-count by 66%. After that you have to calculate the distance between the given location and the locations in the resultset
To get the distance between two geolocation you can also use the geolocation in the package. Therefore the Haversine formula is used.
theStartGeolocation := SimGeolocation fromString: '48.01473,7.8519233'.
theEndGeolocation := SimGeolocation fromString: '48.008112,7.8481903'.
theStartGeolocation distanceTo: theEndGeolocation