1.记录gps信息，定位类型 gps agps ，偏移量
2.根据id检索用户 gps 历史记录
3.创建围栏
4.围栏内用户检索（先实现 圆形和矩形）
5.判断一个点是否进出围栏
应用场景：
o2o应用场景
1.商场推送
2.人群分析，来过这里还去过哪里
包裹自动分配投递站（根据地址获取坐标，根据坐标获取围栏 根据围栏找到对应投递站）
用postgreSQL做基于地理位置的app
http://blog.csdn.net/wusuopubupt/article/details/21621477
| Function | Returns | Description |
|---|---|---|
earth() |
float8 | Returns the assumed radius of the Earth. |
sec_to_gc(float8) |
float8 | Converts the normal straight line (secant) distance between between two points on the surface of the Earth to the great circle distance between them. |
gc_to_sec(float8) |
float8 | Converts the great circle distance between two points on the surface of the Earth to the normal straight line (secant) distance between them. |
ll_to_earth(float8, float8) |
earth | Returns the location of a point on the surface of the Earth given its latitude (argument 1) and longitude (argument 2) in degrees. |
latitude(earth) |
float8 | Returns the latitude in degrees of a point on the surface of the Earth. |
longitude(earth) |
float8 | Returns the longitude in degrees of a point on the surface of the Earth. |
earth_distance(earth, earth) |
float8 | Returns the great circle distance between two points on the surface of the Earth. |
earth_box(earth, float8) |
cube | Returns a box suitable for an indexed search using the cube @> operator for points within a given great circle distance of a location. Some points in this box are further than the specified great circle distance from the location, so a second check using earth_distance should be included in the query. |