Reference
緯度経度を文字列で表すGeoHash - @masuidrive bloghttp://blog.masuidrive.jp/index.php/2010/01/13/geohash/
Wikipedia - Geohash
RubyForge: pr_geohash-1.0.0-released
http://rubyforge.org/forum/forum.php?forum_id=35553
masuidrive's pr_geohash at master - GitHub
http://github.com/masuidrive/pr_geohash