【GPS】 数据围栏

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.