http://www.mullie.eu/geographic-searches/
https://github.com/thephpleague/geotools/blob/master/src/Geohash/Geohash.php
http://www.movable-type.co.uk/scripts/latlong-db.html
http://markmaunder.com/2009/10/10/mysql-gis-extensions-quick-start/
http://dev.mysql.com/doc/refman/5.7/en/spatial-extensions.html
http://lbs.amap.com/yuntu/reference/cloudsearch/
http://developer.baidu.com/map/index.php?title=lbscloud/api/geosearch&qq-pf-to=pcqq.discussion