Charlie Savage has a great post introducing coordinate systems in his blog. I too have had many questions on coordinate systems, projections and datums, and Charlie have made a great introduction to the world of "spatial reference systems". He promises to post some more on the topic, so keep track of his blog if you find these topics confusing (I know I did when I first was told about datums and coordinate systems).

Charlie Savage在他的blog有一个介绍坐标系的great post 。我也有许多关于坐标系，projections and datums（投影和数据？）的问题，Charlie也对spatial reference systems（特殊参考系）做了详细的介绍，他许诺关于这个话题会介绍的更多，如果你对这个话题感到困惑，那么关注他的博客。（我了解我第一次知道datums and coordinate systems时的样子 ）