When a data set is brought into a mapdocument, we will call it a layer.
当资料夹被载入地图文件，我们将称它为图层。
2
The Beijing coordinate system in 1954 and the Xi 'an coordinate system in 1980 are two kinds of coordinate systems kept in the mapdocument presently in China.
1954年北京坐标系和1980西安坐标系是我国目前地图资料中存在的二种坐标系统。
3
Using data structures that don't easily map to a document tends to produce tight coupling.