Encode Geo-hashes with Precision

Encode Geo-hashes with Precision

By looking at the Earth shape, one recognizes the size of geohash cells shrinks by moving away from the equator along an arbitrary latitude. In other words the radius depends on the latitudinal value. In cases where a user should able to define a precision he needs, this fact can be used to reduce the length of generated hashes. Also a precision value is a reasonable alternative to the currently used maxtreelevel value, used in geohash implementations.

An Example

The geohash 7zzzzzz refers to a geohash cell latlon1~(-0.00137, -0.00137) to latlon2=(0, 0) near the equator. The distance between these point corresponds to ~216.19656m. A geohash cell at a pole gzzzzzz with the same hash length refers to a cell with a size up to ~152.87406m. If i.e. a precision of 200m is accurate enough to encode geo-positions one can use hashes of length 8 for points between the latitudes -32.4837° and +32.4837° and geohashes of length 7 otherwise.

Start by locating the geohash encoding implementation and the existing maxtreelevel handling. Use the latitude-dependent cell-size example and the requested precision value to define the encoding behavior, then identify tests for hash length and positional accuracy. Done means a documented, tested precision-based alternative to maxtreelevel.