proj.4 is a standard UNIX filter function which converts geographic longitude and latitude coordinates into cartesian coordinates (and vice versa), and it is a C API for software developers to include coordinate transformation in their own software. proj.4 is maintained on GitHub.
proj.4是一个标准的UNIX过滤函数，将地理经纬度坐标s,t转换到笛卡儿坐标x,y,z（反之亦然），而且它是一个C接口类库，开发者可以引用从而实现他们自己软件中的坐标转换。proj.4在GitHub上保管。
原文：http://www.cnblogs.com/2008nmj/p/6859918.html